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ANATOMY  AND  HISTOLOGY 

OF  THE 

MOUTH  AND  TEETH 


BROOMELL  AND  FISCHELIS 


ANATOMY  AND  HISTOLOGY 


OF  THE 


MOUTH  AND  TEETH 


BY 

* 

I.  NORMAN  BROOMELL,  D.  D.  S. 

DEAN    AND    PROFESSOR    OF    PROSTHETIC     DENTISTRY,    DENTAL    ANATOMY    AND     HISTOLOGY,    DENTAL 
DEPARTMENT,    MEDICO-CHIRURGICAL    COLLEGE    OF    PHILADELPHIA 


AND 


PHILIPP  FISCHELIS,  M.  D. 

ASSOCIATE    PROFESSOR    OF    HISTOLOGY    AND    DEMONSTRATOR    IN    EMBRYOLOGY, 
MEDICO-CHIRURGICAL   COLLEGE    OF    PHILADELPHIA 


FOURTH  EDITION,  REVISED,  WITH   368  ILLUSTRATIONS 


PHILADELPHIA 

P.   BLAKISTON'S   SON' &   CO. 

1012   WALNUT   STREET 
1913 


Copyright,  1913,  by  P.  Blakiston's  Son  &  Co. 


THE . MAPLE. PRESS- YORK- PA 


TO 

C.   N.    PEIRCE,  D.  D.  S. 

AS  A   SOUVENIR   OF   A   LONG  AND   VALUED   FRIENDSHIP  AND  A 

TESTIMONY   OF   ESTEEM   FOR  HIS   PROFESSIONAL 

AND    PRIVATE   WORTH 

THIS  VOLUME  IS  RESPECTFULLY  DEDICATED 

BY    THE   AUTHORS 


PREFACE  TO  THE  FOURTH  EDITION 


In  view  of  the  fact  that  it  has  become  necessary  to  issue  a  new  edition 
of  this  work  in  a  comparatively  short  time,  the  authors  have  taken  ad- 
vantage of  the  opportunity  thus  offered,  to  still  further  revise  and  enlarge 
the  subject  matter  along  the  lines  begun  in  the  third  edition. 

In  that  edition  it  was  suggested  that  "it  has  been  justly  brought 
forward  within  recent  years*  that  there  is  a  lack  of  interest  and  enthu- 
siasm for  collateral  reading,  special  studies,  and  original  investigation 
not  only  among  students,  but  also  among  graduates  in  Dentistry.  The 
experience  gained  in  teaching  has  suggested  to  the  authors  that  this  is  due 
to  the  fact  that  dental  students  use  to  a  great  extent  text-books  on  Anatomy, 
Histology,  and  Embryology,  chiefly  intended  for  medical  students,  and 
they  generally  believe  that  most  of  the  matter  which  is  contained  in  these 
books  is  not  intended  for  them,  having  no  practical  application.  Their 
attention  is  therefore  limited  to  those  chapters  which  seem  of  importance 
to  them,  and  thus  an  intelligent  understanding  of  the  subject  is  not 
obtained.  Recognizing  this  fact,  it  seemed  advisable  to  include  in  this 
Dental  Text-book  a  description  pertaining  to  general  Cytology,  general 
Embryology,  and  Histogenesis,  and  a  further  presentation  of  the  signifi- 
cance of  the  facts  thus  gained  for  an  intelligent  understanding  of  the 
formation  of  the  organs  of  the  mouth." 

In  the  present  edition  this  plan  has  been  further  amplified  by  the 
addition  of 

i.  A  comprehensive  description  of  the  present  status  of  our  knowledge 
of  blood  and  lymph. 

2.  A  comprehensive  description  of  the  structure  of  the  circulatory 
apparatus. 

3.  A  general  consideration  of  the  structure  of  various  forms  of  glands. 
In  addition  to  these  features  the  subject  matter  of  the  various  chapters 

has  been  rearranged  to  conform  with  the  general  plan  of  the  work. 

It  has  been  the  aim  to  present  the  subject  in  a  concise  and  attractive 
form,   including   some   illustrations   which   never  before  appeared  in  a 

*  Proceedings  of  the  Meeting  of  the  Institute  of  Dental  Pedagogics. 

vii 


Vlll  PREFACE    TO    THE    FOURTH   EDITION 

work  designed  for  dental  students.  It  is  the  hope  of  the  authors  that 
the  additions  made  will  help  to  awaken  an  interest  for  collateral  reading 
and  thus  elevate  the  standard  of  education  among  dental  graduates. 

The  authors  acknowledge  their  indebtedness  to  the  publishers  for 
their  kind  assistance  in  adding  new  illustrations  to  further  the  usefulness 
of  the  book. 
Philadelphia. 


PREFACE  TO  THE  FIRST  EDITION. 


In  the  preparation  of  this  work  it  has  been  the  aim  of  the  author  to 
systematically  describe  those  parts  of  human  anatomy  which  come 
directly  under  the  care  of  the  stomatologist.  In  the  earlier  chapters, 
which  are  devoted  to  a  gross  description  of  the  mouth  and  those  tissues 
which  enter  into  its  construction,  there  has  been  no  attempt  at  originality 
other  than  in  the  arrangement,  which  includes  a  complete  description  of 
one  part  before  another  is  taken  up. 

In  the  writing  and  classification  of  the  succeeding  chapters  the  writer 
has  attempted  what  others,  though  wiser  and  better  qualified,  appeared 
unwilling  to  undertake,  and  it  is  from  the  works  of  such  as  these  that  the 
foundation  for  the  present  work  has  been  derived. 

Within  the  last  few  years  the  progress  in  nearly  every  branch  of 
dental  education  has  made  a  work  of  this  character  an  imperative  want. 
Dental  therapeutics  and  dental  chemistry  have  been  well-nigh  recon- 
structed, while  the  investigations  of  the  microscopist  and  physiologist  have 
brought  forth  many  valuable  revelations.  Next  in  importance  has  been 
the  advance  in,  or  rather  the  introduction  of,  technic  teaching.  Con- 
siderable space  has,  therefore,  been  devoted  to  the  surface  anatomy  of  the 
individual  teeth,  with  a  hope  that  it  may  be  of  value  in  dental  anatomy 
technic. 

While  in  one  or  two  instances  the  writer  has  departed  from  the  field 
assigned  as  a  text,  the  parts  thus  included  are  so  closely  associated  with 
the  mouth,  both  in  a  constructive  and  in  a  functional  manner,  that  the 
work  would  be  lacking  in  completeness  if  they  were  omitted. 

The  illustrations  are,  with  but  few  exceptions,  the  original  work  of 
the  author,  being  reproduced  by  photograph  from  the  actual  subject. 
In  many  instances  dissections  were  required  to  reveal  the  parts,  this 
being  particularly  true  of  those  illustrations  included  in  the  chapter  on 
the  Development  of  the  Teeth,  about  one  hundred  dissections  being  re- 
quired to  accomplish  the  purpose.  In  preparing  the  illustrations  de- 
scriptive of  the  various  surfaces  of  the  individual  teeth,  the  progress  of 
the  work  was  materially  interfered  with  by  the  difficulty  experienced  in 
securing  normal  teeth  out  of  the  mouth;  may  their  number  ever  grow  less. 

ix 


X  PREFACE    TO    THE    FIRST    EDITION 

The  author  desires  to  thus  publicly  acknowledge  obligations  to  the 
works  of  Tomes,  Black,  Morris,  Stohr,  Klein,  and  Strieker.  He  is  also 
indebted  to  Prof.  A.  P.  Brubaker  and  to  Dr.  C.  P.  Shoemaker  for  valuable 
assistance  rendered,  and  to  P.  Blakiston's  Son  &  Co.  for  their  many  cour- 
tesies during  the  preparation  of  the  volume. 

That  there  is  a  place  for  such  a  work  as  this  purports  to  be  the  writer 
has  but  little  doubt;  that  the  following  pages  will  fill  that  demand  is  his 
earnest  desire,  and  it  remains  for  the  reader  to  ascertain  how  far  these 
demands  have  been  met  in  the  direction  of  its  aim  and  endeavor. 


TABLE  OF  CONTENTS. 


PART  I.— ANATOMY. 

CHAPTER  I. 

General  Description  of  the  Mouth. — The  Buccal  Orifice;  The  Lateral 
Walls  of  the  Mouth;  The  Hard  Palate,  or  Dome  of  the  Mouth; 
The  Soft  Palate  and  Fauces;  The  Floor  of  the  Mouth;  The 
Tongue  and  its  Attached  Muscles, i 

CHAPTER  II. 

Muscular  Tissues  of  the  Mouth;  of  the  Lips;  of  the  Cheeks;  of  the 
Soft  Palate;  of  the  Tongue, 34 

CHAPTER  III. 

The  Bones  of  the  Mouth. — The  Superior  Maxillae;  The  Palate  Bones; 

The  Inferior  Maxilla,  or  Mandible, 37 

CHAPTER  IV. 

The  Temporomandibular  Articulation. — The  Muscles  of  Mastica- 
tion,       64 

CHAPTER  V. 

General  Description  of  the  Teeth;  The  Permanent  Teeth;  Classifi- 
cation, Surfaces,  etc.;  The  Roots  of  the  Teeth;  The  Dental  Arch,     75 

CHAPTER  VI. 

Occlusion  of  the  Teeth, 88 

CHAPTER  VII. 

The  Blood-  and  Nerve-supply  to  the  Teeth,      94 

CHAPTER  VIII. 

A  Description  of  the  Upper  Teeth  in  Detail. — Calcification,  Erup- 
tion,   and    Average    Measurements;    Their    Surfaces,    Ridges, 

Fossae,  Grooves,  etc., 102 

xi 


Xll  TABLE    OF    CONTENTS 

CHAPTER  IX. 

A  Description  of  the  Lower  Teeth  in  Detail. — Calcification,  Erup- 
tion, and  Average  Measurements;  Their  Surfaces,  Ridges, 
Fossae,  Grooves,  etc.,      164 

CHAPTER  X. 

The  Pulp-cavities  of  the  Teeth, 190 

CHAPTER  XI. 

The  Deciduous  Teeth,  Their  Arrangement,  Occlusion,  etc.;  Calci- 
fication, Eruption,  Decalcification,  Shedding  Process,  and 
Average  Measurements;  Their  Surfaces,  Grooves,  Fossae, 
Ridges,  and  Pulp-cavities, 210 


PART  II.— HISTOLOGY  AND  HISTOGENESIS. 

CHAPTER  I. 

General  Cytology;  General  Embryology  and  Histogenesis,     ....   233 

CHAPTER  II. 

Elementary  Tissues. — Epithelial  Tissue;  Connective  Tissue;  Mus- 
cular Tissue;  Nervous  Tissue;  Blood  and  Lymph, 248 

CHAPTER  III. 
Circulatory  Organs,  Glands, 274 

CHAPTER  IV. 

The  Mucous  Membrane  of  the  Mouth. — Of  the  Lips;  Of  the  Cheeks; 
Of  the  Gums;  Of  the  Roof  of  the  Mouth,  Hard  and  Soft  Palate; 
Of  the  Floor  of  the  Mouth  and  the  Tongue, 283 

CHAPTER  V. 

Other  Structures  Within  the  Mouth.— The   Gums;  The  Mucous 

Membrane;  The  Alveolodental  Membrane, 294 

CHAPTER  VI. 

Glands  and  Ducts  of  the  Mouth. — Of  the  Lips;  Of  the  Cheeks; 
Of  the  Hard  and  Soft  Palate;  Of  the  Tongue;  The  Salivary 
Glands, 305 


TABLE    OF    CONTENTS  Xlll 

CHAPTER  VII. 

Tissues  of  the  Teeth. — Enamel;  Dentin;  Cementum;  The  Tooth- 
pulp;  The  Alveolodental  Membrane,      313 

CHAPTER  VIII. 

Embryology  of  the  Mouth  and  Teeth, 374 

CHAPTER  IX. 

Development  of  the  Teeth. — The  Dental  Germs,  Enamel  Organ, 
and  Dentin  Organ;  The  Dental  Follicle;  Calcification,  Erup- 
tion, etc., 391 

CHAPTER  X. 

Anomalies  of  the  Teeth, 451 

Index, 461 


ANATOMY   AND   HISTOLOGY 


OF    THE 


MOUTH   AND   TEETH 


PART  I.-ANATOMY. 


CHAPTER  I. 
General  Description  of  the  Mouth. — The  Buccal  Orifice  (the  Lips).— 
The  Lateral  Walls  of  the  Mouth  (the  Cheeks).— The  Hard 
Palate,  Dome  or  Roof  of  the  Mouth.— The  Soft  Palate  and 
Fauces. — The  Floor  of  the  Mouth.— The  Tongue  and  its  Attached 
Muscles. 

The  mouth  (Fig.  i)  {stoma,  pi.  stomata)  is  the  entrance  or  gateway 
to  the  alimentary  canal,  and  is  situated  between  the  superior  and  inferior 
maxillary  bones  and  their  attached  tissues.  It  contains  the  active  organs 
of  mastication,  the  teeth,  the  organs  of  taste,  of  which  the  tongue  is  chief, 
together  with  some  of  the  parts  which  assist  in  articulate  speech.  Anato- 
mists usually  divide  this  cavity  into  two  compartments,  the  teeth  serving 
to  separate  one  from  the  other,  the  inner  space  being  called  the  mouth, 
while  that  between  the  teeth  and  lips  or  cheeks  is  known  as  the  vestibule 
of  the  mouth.  In  this  description  all  that  space  bounded  anteriorly  by  the 
lips,  posteriorly  by  the  pillars  of  the  fauces,  and  laterally  by  the  cheeks, 
will  be  considered  as  a  single  cavity,  and  the  organs  and  structures  con- 
tained therein,  together  with  all  parts  directly  interested  in  its  formation, 
will  constitute  a  text  for  this  work.  The  entrance  to  the  cavity  of  the  mouth 
is  formed  by  a  freely  movable  transverse  orifice  or  slit,  the  buccal  orifice, 
while  communication  is  made  with  the  pharynx  posteriorly  through  the 
fauces.  Entering  into  the  construction  of  the  mouth  and  assisting  in  the 
performance  of  its  functions  are  bones,  ligaments,  muscles,  blood-vessels, 
nerves,  glands,  ducts,  etc.,  each  of  which  will  be  described  in  turn. 


2  ANATOMY 

THE  BUCCAL  ORIFICE, 

or  entrance  to  the  cavity  of  the  mouth,  is  a  transverse  opening  somewhat 
variable  in  extent,  the  extremities  of  which  are  known  as  the  corners  or 
angles  of  the  mouth.     The  orifice  is  bounded  by  two  fleshy  folds,  the 


Fig.  i. — Outer  Wall  of  Nasal  Fossa,  with  Mouth,  Pharynx  and  Larynx  in  Vertical 

Section.    {Deaver.) 

_  a,  Superior  meatus;  b,  superior  turbinate  body;  c,  middle  turbinate;  d,  inferior  turbinate; 
e,  inferior  meatus;  g,  tongue;  h,  posterior  pillar  of  fauces;  i,  geniohyoglossus  muscle;  j,  genio- 
hyoid muscle;  k,  hyoid  bone;  /,  mylohyoid  muscle;  m,  thyrohyoid  membrane;  n,  ventricle 
of  larynx;  o,  thyroid  cartilage;  p,  diaphragma  sellae;  q,  cavum  sellae;  r,  sphenoidal  sinus; 
s,  middle  meatus;  t,  rhino  pharynx;  u,  Eustachian  orifice;  v,  hard  palate;  w,  soft  palate; 
x,  uvula;  y,  anterior  pillar  of  fauces;  z,  tonsillar  fossa;  aa,  oropharynx;  bb,  epiglottis;  cc,  ary- 
epiglottic  fold;  dd,  laryngo pharynx;  ee,  suprarimal  portion  of  larynx;  ff,  ventricular  band; 
gg,  vocal  band;  hh,  infrarimal  portion  of  larynx;  it,  cricoid  cartilage;  jj,  tracheal  ring. 

upper  and  lower  lips  (labia),  the  former  usually  being  in  the  form  of  a 
double  curve,  coming  together  at  the  median  line  and  forming  a  small 
teat  or  tubercle,  while  the  latter  is  made  up  of  a  single  curve  extending 
from   angle   to  angle.     While   this   general   description   applies   to  the 


THE    BUCCAL    ORIFICE  3 

labial  forms  most  frequently  met  with,  this  must  not  be  taken  for  a  con- 
stant condition.  In  some  instances  the  lips  are  thin,  with  straight  paral- 
lel margins,  firmly  set  against  the  teeth,  and  seldom  separated  from 
each  other  when  at  rest.  In  another  class  they  are  thick,  full,  and  promi- 
nent, with  their  margins  strongly  curved,  resting  lightly  against  the  teeth, 
and  more  or  less  separated  from  each  other  during  rest.  Accompanying 
these  extremes  as  well  as  the  intervening  conditions  are  various  other 
peculiarities,  such  as  the  color,  the  rigidity  or  flexibility  of  the  muscular 
structure,  etc.  The  upper  lip  generally  overhangs  the  lower,  but  in  some 
instances  the  lower  lip  is  the  most  prominent.  Externally  the  lips  are 
covered  by  the  common  integument,  internally  and  over  their  contiguous 
surfaces  by  a  continuation  of  the  integument,  the  mucous  membrane. 
Between  the  external  and  internal  coverings  and  forming  the  substance 
of  these  fleshy  folds  are  muscular  fibers  in  which  are  imbedded  numerous 
blood-vessels,  nerves,  and  glands  (labial  glands).  By  the  various  muscles 
which  enter  into  their  construction  the  lips  are  loosely  attached  to  the 
surfaces  of  the  maxillary  bones. 

The  integument,  or  external  covering  of  the  lips,  is  similar  to  the 
skin  covering  other  parts  of  the  body.  In  the  male  it  is  subject  to  a 
peculiar  change  and  modification  of  its  outer  layer,  resulting  in  the  pro- 
duction of  a  hairy  growth. 

The  mucous  membrane,  or  internal  covering  of  the  lips,  the  begin- 
ning of  which  is  strongly  manifest  by  its  bright-red  color,  is  without  moist- 
ure on  the  contiguous  surfaces,  is  extremely  sensitive,  and  contains  a 
number  of  vascular  papillae,  many  of  which  are  accompanied  by  nerve 
terminals.  Mucous  membranes  are  described  as  lining  certain  cavities  or 
tracts,  as  the  digestive  tract,  the  respiratory  tract,  and  the  genito-urinary 
tract,  and  it  is  upon  the  contiguous  surfaces  of  the  lips  that  the  digestive 
tract  begins.  The  line  of  junction  between  the  integument  and  the  mu- 
cous membrane  is  quite  variable  in  form,  but  usually  corresponds  to  the 
general  curvature  of  the  lips.  Internally  at  the  median  line  each  lip  is 
provided  with  a  pronounced  fold  of  mucous  membrane,  which  is  attached 
to  the  basal  portion  of  the  gum,  the  frenum  of  the  lip  (fraenum  labium 
superioris  and  inferioris),  which,  in  a  measure,  check  the  movements 
of  the  lips. 

Muscles  of  the  Lips. 

The  muscular  fibers  within  the  substance  of  the  lips  are  principally 
those  of  a  single  muscle,  the  orbicularis  oris,  but  associated  with  it  is  a 
portion  of  the  elevator  and  depressor  muscles  of  the  lips,  the  levator 


4  ANATOMY 

labii  superior  is  alceque  nasi,  levator  labii  superior  is,  depressor  labii  infer i- 
oris  or  quadratus  mcnti,  and  the  zygomaticus  minor. 

Orbicularis  Oris. — This  is  the  sphincter  muscle  which  surrounds 
and  controls  the  buccal  orifice.  In  form  it  is  an  oval  sheet  with  the 
long  axis  placed  transversely,  the  fibers  being  continued  from  one  lip  to 
the  other  by  passing  around  the  angles  of  the  mouth.  It  is  divided  into 
an  interna]  or  labial  portion,  and  an  external  or  facial  portion.  The 
labial  portion  forms  the  red  part  of  the  lips,  and  has  no  bony  attachment 
except  through  the  medium  of  the  adjacent  muscles.  The  external  or 
facial  portion  forms  the  deeper  layer  and  blends  with  the  surrounding 
muscles,  works  in  conjunction  with  them,  and  is  provided  with  the  follow- 
ing small  bony  attachments :  The  nasolabial  slips  are  attached  to  the 
septum  of  the  nose,  other  fibers  are  attached  to  the  incisive  fossa  of  the 
superior  maxilla  over  the  position  of  the  lateral  incisor  tooth,  and  to  the 
incisive  fossa  of  the  inferior  maxilla  near  the  socket  of  the  lateral  incisor 
or  cuspid  tooth. 

Structure. — The  muscle  consists  of  three  sets  of  fibers,  one  of  which 
runs  transversely,  one  in  a  vertical,  and  one  in  an  anteroposterior  direc- 
tion. The  transverse  set  is  continuous  with  the  fibers  of  the  buccinator 
or  cheek  muscle,  and  forms  the  greater  part  of  the  muscle.  The  red  or 
labial  portion  of  the  muscle  is  also  formed  from  the  same  fibers,  while  the 
vertical  fibers  form  the  superficial  part  of  the  facial  portion  and  are  con- 
tinuous with  the  fibers  of  the  levator  and  depressor  muscles.  Some  of 
these  latter  fibers  pass  around  the  corners  of  the  mouth,  thus  becoming 
transverse,  those  from  above  passing  to  the  lower  lip,  while  those  from 
below  pass  to  the  upper  lip.  The  anteroposterior  fibers  pass  from  before 
backward  between  the  transverse  fibers,  and  unite  the  mucous  mem- 
brane to  the  skin.  These  are  chiefly  found  in  the  labial  portion  of  the 
muscle. 

Relations. — The  inner  margin  of  the  superficial  surface  is  closely 
connected  with  the  integument,  while  superimposed  between  this  and  the 
outer  portion  is  a  layer  of  fatty  tissue.  Upon  the  deep  surface  lies  the 
mucous  membrane  of  the  mouth,  separated  from  the  muscular  tissue  by 
blood-vessels,  the  mucous  glands,  and  small  salivary  glands. 

Action.- — To  bring  the  lips  together,  to  draw  the  upper  lip  downward, 
and  the  lower  lip  upward;  to  draw  together  the  corners  of  the  mouth; 
to  throw  both  lips  outward;  to  draw  them  back  against  the  teeth,  and  to 
oppose  the  action  of  all  other  muscles  that  blend  into  it  and  inclining  to 
draw  it  in  various  directions. 

Levator  Labii  Superioris  Alaeque  Nasi. — As  its  name  implies,  this 


THE    BUCCAL    ORIFICE 


muscle  is  an  elevator  of  the  upper  lip  and  the  wing  of  the  nose.  It  is  one 
of  the  superficial  facial  muscles,  is  thin  and  triangular,  and  is  situated 
by  the  side  of  the  nose,  extending  from  the  infra-orbital  ridge  to  the 
upper  lip. 


Galea  apo- 
neurotiea 


Orbicularis  oeuli 


Procerus 
Quadr.  labii  sup. 
pars  angularis 
Nasalis,  pars  transversa 
Dilator  naris  anterior- 
Dilator  naris  posteriori 
Quadr.  labii  sup. 
caput  infraorbitale 

Caput  zygomaticum 

Caninus 

Orbicularis  oris 


Quadratus  labii  inferioris 
Triangularis 


Aurieularis 
superior 


Aurieularis 
anterior 


Occipitalis 


Aurieularis 
posterior 


Fig.  2. — The  Superficial  Muscles  of  the  Head  and  Neck.     (Morris.) 


Origin. — From  the  nasal  process  of  the  superior  maxilla  near  its 
orbital  margin. 

Insertion. — From  its  origin  it  passes  almost  directly  downward, 
dividing  into  two  portions,  the  smaller  of  which  is  inserted  into  the  nasal 
wing,  while  the  larger  portion  is  prolonged  downward,  blending  into  the 


6  ANATOMY 

orbicularis  oris  and  levator  labii  superioris,  and  forming  a  part  of  the 
substance  of  the  upper  lip. 

Relations. — Superficially,  by  the  integument;  deeply,  by  the  levator 
anguli  oris  and  compressor  narium. 

Action. — By  its  smaller  and  shorter  portion  to  raise  the  wing  of  the 
nose  and  to  dilate  the  nostril;  by  its  larger  and  longer  portion  to  elevate 
the  inner  half  of  the  upper  lip. 

Levator  Labii  Superioris. — This  muscle  belongs  to  the  superficial 
layer,  and  derives  its  name  from  its  action. 

Origin. — From  the  facial  surface  of  the  superior  maxilla,  at  a  point 
between  the  orbital  cavity  and  the  infra-orbital  foramen.  Also  by  the 
attachment  of  a  few  fibers  to  the  malar  bone. 

Insertion. — Passing  downward  and  inward,  it  is  inserted  into  the  orbic- 
ularis oris  and  the  integument  of  the  upper  lip.  Near  its  lower  third 
it  joins  the  levator  labii  superioris  alaeque  nasi,  and  acts  in  conjunction 
with  it.  Occasionally  it  is  reinforced  by  fibers  from  the  orbicularis  pal- 
pebrarum, which  it  receives  at  its  outer  border. 

Relations. — Superficially,  by  the  orbicularis  palpebrarum  and  the 
integument;  deeply,  by  the  levator  anguli  oris;  the  compressor  nasi  at 
its  origin,  and  by  the  infra-orbital  vessels  and  nerves. 

Action. — To  elevate  the  upper  lip. 

Levator  Labii  Inferioris. — This  muscle,  also  known  as  levator 
menti,  lies  immediately  beneath  the  mucous  membrane  of  the  lower 
lip,  and  can  best  be  dissected  by  everting  the  lip  and  lifting  off  the 
membrane. 

Origin. — From  the  incisor  fossa  of  the  lower  jaw  at  its  upper  border. 

Insertion. — Into  the  integument  of  the  skin. 

Relations. — With  the  labial  integument,  the  lower  border  of  the  orbic- 
ularis oris  muscle,  and  its  superficial  surface  with  the  oral  mucous  mem- 
brane. Deeply  it  is  in  close  contact  with  the  periosteum  and  the  depres- 
sor labii  inferioris. 

Action. — To  raise  and  cause  to  protrude  the  integument  of  the  chin. 

Depressor  Labii  Superioris. — This  small  muscle  with  its  fellow  of 
the  opposite  side  is  sometimes  found  within  the  mucous  membrane  form- 
ing the  frenum  of  the  upper  lip. 

Origin. — It  arises  from  the  incisive  fossa  along  its  lower  margin  and 
some  of  its  fibers  are  attached  to  that  part  of  the  alveolar  process  closely 
associated  with  the  fossa. 

Insertion. — From  the  point  of  origin  the  fibers  pass  upward  and  are 
attached  to  the  lower  border  of  the  nostrils  and  partition  of  the  nose. 


THE    BUCCAL    ORIFICE  7 

Some  of  the  fibers  of  this  muscle  are  also  attached  to  the  integument  cover- 
ing the  wing  of  the  nose.  The  balance  pass  downward  and  mingle  with 
the  fibers  of  the  muscular  structure  of  the  upper  lip. 

Relations. — At  its  point  of  origin  the  fibers  are  closely  associated  with 
the  mucous  membrane  forming  the  gums.  Above  this  the  muscular 
structure  of  the  lip  overlies  these  fibers.  Deeply  it  rests  upon  the  surface 
of  the  superior  maxilla,  and  joins  its  fellow  of  the  opposite  side  at  the 
median  line. 

Action. — To  depress  the  upper  lip. 

Depressor  Labii  Inferioris,  or  Quadratus  Menti. — The  name 
of  this  muscle  is  derived  from  its  form  and  action.  It  belongs  to  the 
second  layer  of  facial  muscles,  is  quadrilateral  in  shape,  and  consists  of 
parallel  fibers  which  meet  above  in  the  median  line. 

Origin. — At  the  outer  aspect  of  the  lower  border  of  the  inferior  maxilla, 
from  a  point  near  the  symphysis  to  the  space  beneath  the  first  bicuspid 
tooth. 

Insertion. — Its  fibers  pass  upward  and  inward,  and  after  uniting 
with  its  fellow  of  the  opposite  side,  blend  into  the  body  of  the  orbicularis 
oris  of  the  lower  lip. 

Relations. — By  its  superficial  surface  with  the  integument  and  a 
portion  of  the  depressor  anguli  oris;  deeply,  with  the  mental  nerve  and 
vessels,  a  portion  of  the  orbicularis  oris,  the  mucous  membrane  lining  the 
lower  lip,  and  the  labial  glands. 

Action. — To  draw  down  and  somewhat  evert  the  lower  lip. 

Zygomaticus  Minor. — An  extremely  slender  muscle  belonging  to  the 
superficial  set  of  facial  muscles.  It  is  closely  associated  with  a  larger 
muscle,  the  zygomaticus  major,  belonging  to  the  angular  series,  to  be 
described  in  connection  with  the  muscles  of  the  cheek. 

Origin. — From  the  anterior  inferior  part  of  one  of  the  facial  bones 
— the  malar — close  to  its  junction  with  the  superior  maxilla. 

Insertion. — It  passes  downward  and  forward,  its  fibers  becoming 
lost  in  the  special  elevator  muscle  of  the  upper  lip  about  midway  between 
the  median  line  and  the  angle  of  the  mouth. 

Relations. — Superficially,  by  the  integument,  by  its  deep  surface  with 
the  levator  anguli  oris,  facial  portion  of  the  orbicularis  oris,  and  the  infra- 
orbital branch  of  the  facial  nerve. 

Action. — To  elevate  and  somewhat  evert  the  upper  part  of  the  lip. 

The  Blood-supply  to  the  Lips. 

The  blood-supply  to  the  lips  is  principally  through  the  superior  and 
inferior  coronary  arteries,  both  of  which  are  branches  of  the  facial  artery. 


8  ANATOMY 

In  addition  to  these  the  inferior  labial  artery  and  the  sub-mental,  also 
branches  of  the  facial,  and  the  mental  branch  of  the  inferior  dental  artery 
supply  a  part  of  the  lower  lip. 

The  superior  coronary  artery  courses  along  the  inferior  margin 
of  the  upper  lip,  between  the  mucous  membrane  and  the  fibers  of  the  or- 
bicularis oris  muscle.  At  the  median  line  it  anastomoses  with  its  fellow 
of  the  opposite  side. 

The  interior  coronary  artery,  somewhat  smaller  than  the  superior, 
supplies  the  lower  lip  by  coursing  through  its  substance  in  a  manner  simi- 
lar to  the  superior  coronary  and  also  anastomoses  with  its  fellow  of  the 
opposite  side  at  the  median  line. 

Course  of  the  Blood  from  the  Heart  to  the  Lips. — From  the 
heart  to  the  aorta,  to  the  common  carotid,  to  the  external  carotid,  to  the 
facial,  to  the  superior  and  inferior  coronary  and  the  inferior  labial  arteries. 
After  passing  through  the  labial  capillaries  the  blood  is  returned  to  the 
heart  through  the  superior  and  inferior  coronary  veins,  and  the  larger  veins 
of  which  they  are  branches. 

Nerves  of  the  Lips. 

The  general  nerve-supply  to  the  lips  is  principally  by  small  branches 
of  the  infra-orbital  nerve  for  the  upper  lip,  and  by  branches  of  the  mental 
nerve  for  the  lower  lip.  The  buccal  and  superior  maxillary  branches  of 
the  lower  division  of  the  facial  nerve  supply  the  orbicularis  oris  muscle; 
the  upper  division  of  the  facial  nerve  sends  branches  which  supply  the 
levator  labii  superioris  alseque  nasi,  as  well  as  the  levator  labii  superioris 
and  the  zygomaticus  minor,  while  the  superior  maxillary  branch  of  the 
lower  division  of  the  facial  supplies  the  depressor  labii  inferioris. 

THE  LATERAL  WALLS  OF  THE  MOUTH. 

The  Cheeks  {buccce). 

The  cheeks  are  continuous  with  and  similar  in  structure  to  the  lips, 
being  covered  internally  by  mucous  membrane  and  externally  by  the  com- 
mon integument.  Immediately  beneath  the  mucous  membrane  are  a 
number  of  transverse  muscular  fibers,  covered  externally  by  a  layer 
of  subcutaneous  fat,  and  lying  between  this  and  the  integument  other 
muscular  tissue,  the  fibers  of  which  radiate  in  various  directions,  accord- 
ing to  the  action  of  the  muscle  to  which  they  belong.  Besides  muscular 
and  fatty  tissue,  there  are  imbedded  within  the  substance  of  the  cheek 
blood-vessels,  nerves,  and  glands.     The  fatty  tissue  spoken  of  as  interven- 


THE    LATERAL    WALLS    OF    THE    MOUTH  9 

ing  between  the  muscular  fibers  gives  to  the  cheek  its  fullness  and 
rotundity. 

The  integument,  or  external  covering  of  the  cheek,  is  similar  in 
structure  to  the  skin  covering  other  parts  of  the  body,  and,  like  the  lips 
in  the  male,  is  productive  of  a  hairy  growth. 

The  mucous  membrane,  or  internal  covering  of  the  cheek,  is  similar 
to  that  of  the  lips,  containing  numerous  glands  {buccal  glands)  which  are 
almost  identical  to,  but  smaller  than,  the  labial  glands.  In  addition  to  the 
buccal  glands  which  are  distributed  over  the  entire  membrane,  there  are 
about  five  of  larger  size,  which  open  into  the  mouth  in  the  region  of  the 
molar  teeth,  and  are  called  molar  glands  (see  Glands  of  the  Mouth). 

The  Muscles  of  the  Cheeks  (See  Fig.  3). 

The  transverse  muscular  fibers  referred  to  as  being  immediately 
beneath  the  mucous  membrane  are  those  of  the  buccinator,  a  muscle 
named  from  its  action,  that  of  being  the  chief  muscle  employed  by  the 
trumpeter.  External  to  the  buccinator  is  the  masseter,  one  of  the  muscles 
of  mastication,  the  elevator  and  depressor  muscles  of  the  angle  of  the 
mouth,  the  levator  anguli  oris,  and  the  depressor  anguli  oris,  and  the 
dermal  muscles,  zygomaticus  major  and  zygomaticus  minor,  and  the 
risorius. 

Buccinator. — This  muscle  forms  the  greater  portion  of  the  lateral 
wall  of  the  mouth.  It  is  deep-seated  in  the  cheek,  being  one  of  the  third 
stratum  of  facial  muscles. 

Origin. — The  fibers  are  distinct  in  their  origin  from  a  part  of  the  alve- 
olar process  of  the  superior  maxillary  bone,  at  a  point  immediately  over 
the  second  and  third  molar  teeth,  from  the  anterior  border  of  the  pterygo- 
maxillary  ligament,  a  narrow  band  of  tendinous  fibers  or  raphe  extend- 
ing from  the  pterygoid  plate  of  the  sphenoid  bone  to  the  mylohyoid  ridge 
of  the  inferior  maxilla  near  the  position  of  the  third  molar  tooth.  Some 
of  its  fibers  also  arise  from  the  outer  wall  of  the  alveolar  process  of  the 
inferior  maxilla  below  the  second  and  third  molars. 

Insertion. — The  fibers  pass  forward  and  converge  as  they  reach  the 
lateral  margins  of  the  orbicularis  oris;  here  the  fibers  of  the  upper  portion 
pass  downward  and  blend  into  the  muscles  of  the  lower  lip,  while  the 
lower  fibers  pass  upward  and  blend  into  those  of  the  upper  lip.  Those 
fibers  which  arise  from  the  inferior  maxilla  pass  forward  and  also  blend 
into  the  lower  lip. 

Relations. — Superficially,  by  the  skin  and  subcutaneous  fat,  the  duct 
of  Stenson,  the  masseter  muscle,  a  portion  of  the  angular  group,  and  the 


IO 


ANATOMY 


facial  arterv  and  vein.  Passing  over  it  are  branches  of  the  facial  and  buc- 
cal nerves,  also  a  layer  of  deep  fascia  continuous  with  that  which  covers 
the  upper  part  of  the  pharynx.  By  its  deep  surface  it  is  in  relation  with 
the  mucous  membrane  and  buccal  glands. 


Corrugator 


Quadr.labii  sup. 
caput  augularis 

Caput  infra- 
orbitale 

Nasalis,  pars 

transversa     /        •^SaSl 

Caninus  *  ■■  ■  » ■»■■ 
Depressor_  ^/^^C  (  f 
se-pti  n...-,:        ^Jf. "'<  i  !. 

Nasalis,  par=  — /Nfe?  -""SI  1 1, 

alaris  r^*S^ 

Orbicularis  oris \^     -iv^    N\ 

Buccinator 


Triangularis 

Quadratus  la- 

bii  inferioris 

Men  talis 

Mylo-hyoid 

Anterior  belly 

of  digajtric 


.-j Temporal 


Posterior  belly 
of  digastric 


Splenius  capitis 


Scalenus  anterior 


FlG-  3.— The  Deeper  Muscles  of  the  Face  and  Neck.     (Morris.) 

Action— -To  draw  outward  or  backward  the  angles  of  the  mouth,  thus 
enlarging  the  buccal  orifice  and  pressing  the  lips  tightly  against  the 
teeth;  to  force  the  food  between  the  occlusal  surfaces  of  the  molar  and 
bicuspid  teeth  during  mastication;  to  diminish  the  concavity  of  the  cheek, 
compressing  the  air  contained  therein  and  forcing   it  forward.     It  be- 


THE    LATERAL    WALLS    OF    THE    MOUTH  II 

comes  an  auxiliary  in  deglutition  by  shortening  the  cavity  of  the  pharynx 
from  before  backward,  through  its  connection  with  the  superior 
constrictor. 

Masseter. — This  muscle  is  placed  immediately  external  to  the 
buccinator,  and  is  one  of  the  principal  muscles  of  mastication.  It  is 
short,  thick,  and  somewhat  quadrate  in  form,  and  is  composed  of  two 
sets  of  fibers,  superficial  and  deep.  The  fibers  of  the  former  are  directed 
obliquely  downward  and  backward;  those  of  the  latter,  which  are  much 
shorter,  pass  almost  vertically  downward. 

Origin. — The  superficial  layer,  from  the  malar  process  of  the  supe- 
rior maxilla,  and  from  the  anterior  portion  of  the  zygomatic  arch  of  the 
malar  bone.  The  deep  layer  from  the  posterior  third  of  the  zygomatic 
arch,  as  well  as  from  the  greater  part  of  its  inner  surface. 

Insertion. — The  superficial  fibers  are  inserted  into  the  ramus  and 
angle  of  the  inferior  maxilla,  and  the  deep  fibers  into  the  upper  half 
of  the  outer  surface  of  the  ramus. 

Relations. — By  its  external  surface  with  the  zygomaticus  major, 
risorius,  and  platysma  myoides  muscles,  the  parotid  gland  and  its  duct; 
by  the  transverse  facial  artery,  the  facial  vein,  and  facial  nerve,  and  by 
the  integument.  By  its  internal  surface  with  the  ramus  of  the  inferior 
maxilla,  a  mass  of  fat  which  separates  it  from  the  buccinator,  and  with  the 
temporal  muscle.  Its  posterior  margin  is  in  relation  with  the  parotid 
gland,  and  its  anterior  with  the  facial  artery  and  vein. 

Action. — The  principal  action  of  this  muscle  is  to  close  the  jaw  and 
to  draw  it  slightly  forward.  (For  further  description,  see  Muscles  of 
Mastication,  part  I,  chap.  iv. 

The  Angular  Series. — The  remaining  muscles  of  the  cheek  are 
those  of  the  angular  series,  or  those  muscles  which  are  inserted  into  the 
angle  of  the  mouth,  two  coming  obliquely  from  above — the  levator  anguli 
oris  and  the  zygomaticus  major — one  running  almost  horizontally  for- 
ward— the  risorius — and  one  ascending  from  below — the  depressor 
anguli  oris. 

Levator  Anguli  Oris. — This  muscle,  which  receives  its  name  from 
its  action,  belongs  to  the  second  layer  of  facial  muscles.  It  is  formed  in 
the  shape  of  a  triangular  sheet. 

Origin. — From  the  canine  fossa  of  the  superior  maxilla,  immediately 
below  the  infra-orbital  foramen. 

Insertion. — Passing  downward  and  outward  it  is  inserted  into  the 
angle  of  the  mouth,  its  fibers  blending  with  those  of  the  orbicularis  oris 
and  the  other  angular  muscles. 


12  ANATOMY 

Relations. — Superficially,  with  the  levator  labii  superioris,  the  zygo- 
matics minor,  and  the  infra-orbital  vessels  and  nerves;  deeply,  with  the 
facial  portion  of  the  orbicularis  oris  and  buccinator  muscles,  and  the 
mucous  membrane  of  the  mouth. 

Action. — Especially  to  elevate  the  angle  of  the  mouth,  and  to  assist  in 
drawing  these  angles  inward,  decreasing  the  size  of  the  buccal  orifice. 

Zygomaticus  Major. — This  muscle,  the  companion  of  which  has 
been  described  in  connection  with  the  muscles  of  the  lips,  belongs  to  the 
first  facial  layer.  It  is  composed  of  a  long,  fleshy  band  of  muscular  fibers, 
which  run  direct  from  their  point  of  origin  to  their  point  of  insertion. 

Origin. — From  the  malar  bone,  in  close  proximity  to  the  zygomatic 
suture. 

Insertion. — From  its  origin  it  passes  obliquely  downward  to  the 
angle  of  the  mouth,  and  blends  into  the  fibers  of  the  orbicularis  oris  and 
depressor  anguli  oris. 

Relations. — Superficially,  with  the  skin  and  subcutaneous  fat;  deeply, 
with  the  malar  bone,  the  masseter,  and  buccinator  muscles,  the  facial 
and  transverse  facial  arteries,  the  facial  vein,  and  branches  of  the  facial 
nerve. 

Action. — To  draw  upward  and  outward  the  angles  of  the  mouth, 
as  in  smiling  or  laughing.  By  contracting,  it  throws  into  prominence  the 
cheek  tissues  in  front  of  the  malar  bone,  and  forces  the  lower  eyelid  up- 
ward. When  acting  simultaneously  with  its  fellow  of  the  opposite  side, 
the  buccal  aperture  is  widened,  and  the  upper  lip  is  elevated,  exposing 
the  superior  teeth. 

Risorius. — One  of  the  superficial  set  of  facial  muscles,  receiving  its 
name  from  its  supposed  action  in  laughter  {rider e,  to  laugh).  It  is  flat 
and  ribbon-shaped,  and  is  frequently  very  small  and  poorly  developed. 

Origin. — From  the  deep  fascia  covering  the  masseter  muscle  and 
parotid  gland,  some  of  its  fibers  occasionally  arising  from,  the  mastoid 
process  of  the  temporal  bone. 

Insertion. — Passing  transversely  forward  and  inward  to  the  angle  of 
the  mouth,  its  fibers  blend  with  those  of  the  orbicularis  oris,  and  the 
depressor  anguli  oris. 

Relations. — Superficially,  with  the  integument  and  subcutaneous  fat; 
deeply,  with  the  masseter  and  buccinator  muscles,  the  facial  artery  and 
vein,  and  branches  of  the  facial  nerve. 

Action. — To  draw  the  angles  of  the  mouth  directly  outward,  thereby 
increasing  the  width  of  the  buccal  orifice. 

Depressor  Anguli  Oris. — Also  one  of  the  superficial  layers  of  facial 


THE    LATERAL    WALLS    OF    THE    MOUTH  1 3 

muscles,  deriving  its  name  in  accordance  with  its  action  upon  the  angle 
of  the  mouth.  It  is  a  triangular-shaped  muscle  with  its  base  below,  be- 
coming narrow  as  it  ascends. 

Origin. — From  the  lower  border  of  the  inferior  maxilla,  and  from  its 
external  oblique  line  below  the  cuspid,  bicuspid,  and  first  molar  teeth. 

Insertion. — Passing  upward  and  inward  it  is  inserted  into  the  integu- 
ment at  the  angle  of  the  mouth,  its  fibers  blending  into  those  of  the  muscles 
previously  described  as  coming  together  at  this  point. 

Relations. — Externally,  with  the  integument;  deeply  or  internally, 
with  the  depressor  labii  inferioris,  the  buccinator,  and  the  inferior  coro- 
nary artery. 

Action. — To  draw  down  the  angle  of  the  mouth  and  to  slightly  ex- 
tend it. 

Blood-supply  to  the  Cheeks. 

The  blood-supply  to  the  cheeks  is  principally  through  the  facial 
artery  and  its  direct  branches,  the  superior  and  inferior  coronary,  the 
transverse  facial,  and  branches  from  the  internal  maxillary. 

The  Facial  Artery  and  Branches. — The  facial  artery,  also  called 
the  external  maxillary,  enters  the  cheek  after  passing  over  the  body  of 
the  inferior  maxilla  at  the  anterior  edge  of  the  masseter  muscle.  It 
courses  obliquely  forward  and  upward  through  the  substance  of  the  cheek, 
until  it  reaches  the  inner  angle  or  canthus  of  the  eye,  where  it  joins  the 
nasal  branch  of  the  ophthalmic  artery,  and  is  called  the  angular  artery. 
Near  the  center  of  the  cheek  the  inferior  labial  artery  is  given  off,  which 
passes  forward  and  downward  to  the  lower  lip,  but  supplies  a  portion  of 
the  cheek  in  so  doing.  Midway  between  the  center  of  the  cheek  and  the 
angle  of  the  mouth  the  superior  and  inferior  coronary  arteries  are  given  off, 
supplying  that  part  of  the  cheek  immediately  adjacent  to  the  angle  of  the 
mouth,  after  which  they  pass  on  to  supply  the  upper  and  lower  lips.  The 
masseteric  branch  is  given  off  in  the  immediate  center  of  the  cheek,  at  a 
point  immediately  below  the  inferior  labial,  passes  directly  upward  over 
the  masseter  muscle,  and  anastomoses  with  branches  of  the  internal 
maxillary  and  transverse  facial.  There  are  also  given  off  from  the 
main  trunk  near  the  center  of  the  cheek  the  buccal  branches,  which  pass 
upward  over  the  buccinator  muscle,  and  also  anastomose  with  branches 
of  the  internal  maxillary  and  transverse  facial  arteries. 

The  Transverse  Facial  Artery. — This  is  the  largest  branch  of  the 
temporal  artery.  It  is  at  first  deeply  seated  in  the  substance  of  the  pa- 
rotid gland,  after  leaving  which  it  courses  transversely  over  and  supplies 


14 


ANATOMY 


the  masseter  muscle,  sends  off  small  branches  which  supply  the  integu- 
ment of  the  cheek,  and  anastomoses  with  the  buccal,  infra-orbital,  and  the 


Orbicularis  oeuli  muscle 


Transverse  facia!  artery 

Zygomaticus  minor 
muscle 

Zygomaticus  major 
muscle 


Buccinator  muscle 
Masseteric  branch 

Masseter 


Stylo-pharyngeus 
muscle 
Stylo-glossus  muscle 

Ascending  palatine 

branch 
Tonsillar  branch 

Externa!  maxillary 


External  carotid 
artery 
Posterior     belly     of 
digastric  muscle 

Lingual  artery 


Frontal  branch  of  ophthalmic 

artery 
Nasal  branch  of  ophl/talmic 

artery 


Angular  nr'ery 
Levator  labii  super- 

ioris  alaeque    nasi 

muscle 
In/,  aorbifal  arten/ 
Levator  labii  super- 

ioris  proprius 
Lateral  natal  artery 
Caninus  muscle 
Artery  of  septum 
Svperwr  labial 

artery 


-  Risorius  muscle 


Inferior  labial  artery 


_  Mental  branch  of  inferior 
alveolar  arteru 
Quadratus  labii  inferioris 

muscle 
Inferior  labial  artery 
Triangularis  muscle 

Submental  artery 
Branches  to  submaxillary 
gland 

Anterior  belly  of  digastric  muscle 
Mylo-hyoid  muscle 


Hyo-glossus  muscle 


HYPOGLOSSAL  NERVE 


Scheme  of  the  Right  External  Maxillary  Artery.     {Morris  after  Walsham.) 


facial  arteries.  Besides  the  arteries  already  named,  the  deeper  portions 
of  the  cheek  receive  blood  from  two  branches  of  the  internal  maxillary 
artery,  the  masseteric  branch  and  the  buccal  branch.     The  former  supplies 


THE    INTERIOR    OF    THE    MOUTH  1 5 

the  masseter  muscle  and  anastomoses  with  the  masseteric  branch  of  the 
facial,  while  the  latter  supplies  the  buccinator  muscle  and  anastomoses 
with  the  buccal  branches  of  the  facial. 

Course  of  the  Blood  from  the  Heart  to  the  Cheeks. — From  the 
heart  to  the  aorta,  to  the  common  carotid,  to  the  external  carotid,  to 
the  facial  and  its  direct  branches,  or  from  the  external  carotid  to  the  tem- 
poral, to  the  transverse  facial  and  branches. 

From  the  cheeks  the  blood  is  returned  to  the  heart  principally  through 
the  facial  vein,  a  division  of  the  anterior  superficial  vein.  It  enters 
the  cheek  at  a  point  midway  between  the  lower  eyelid  and  the  wing 
of  the  nose,  passes  obliquely  downward,  being  in  close  contact  with  the 
anterior  edge  of  the  masseter  muscle  over  the  body  of  the  lower  jaw,  join- 
ing the  internal  jugular  vein  in  the  neck.  The  transverse  facial  vein 
which  follows  the  course  of  the  transverse  facial  artery,  and  the  supe- 
rior and  inferior  coronary  veins  also  collect  and  convey  a  portion  of  the 
blood  from  the  cheeks  to  the  larger  veins  and  thence  to  the  heart. 

Nerves  of  the  Cheeks. 

The  nerve-supply  to  the  cheeks  is  principally  from  the  seventh  or 
facial  nerve  and  its  branches,  the  buccal  branch  supplying  a  greater  part 
of  their  substance.  There  are  also  a  few  fibers  of  the  infra-orbital  branch 
of  the  seventh  nerve  distributed  to  the  labiobuccal  region.  The  buccal 
branch  of  the  lower  division  of  the  facial,  also  the  buccal  branch  of  the 
inferior  maxillary  division  of  the  fifth  nerve,  supplies  the  buccinator 
muscle.  The  infra-orbital  branch  of  the  upper  division  of  the  facial  nerve 
supplies  the  zygomaticus  major  and  the  levator  anguli  oris;  the  buccal 
branch  supplies  the  risorius,  and  the  supramaxillary  branch  of  the  lower 
division  of  the  facial  nerve  supplies  the  depressor  anguli  oris. 

THE  INTERIOR  OF  THE  MOUTH. 

For  convenience  of  description  the  mouth  may  be  divided  into  two 
parts — a  superior  portion  and  an  inferior  portion.  In  dissection  this 
division  may  be  accomplished  by  an  incision  beginning  at  the  angles  of 
the  mouth  and  carried  backward  and  slightly  upward  through  the  sub- 
stance of  the  cheeks  until  the  temporomandibular  articulation  is  reached. 
After  disarticulating  this  joint,  another  incision  is  made,  beginning  at  the 
joint  on  either  side,  carried  downward  and  forward,  then  obliquely  across 
the  throat,  until  the  two  come  together  at  the  median  line.  This  latter 
incision  must  be  deep  enough  to  completely  sever  the  tissues  of  the  throat. 

The  superior  portion  of  the  mouth  contains  the  hard  palate,  or  roof 


1 6  ANATOMY 

of  the  mouth,  the  soft  palate,  and  the  sixteen  upper  teeth,  firmly  set  in  the 
bone  and  surrounded  by  a  dense  fibrous  tissue — the  gums.  The  inferior 
portion  contains  the  tongue  and  its  attached  muscles,  forming  the  floor 
of  the  mouth,  the  sixteen  lower  teeth  and  the  gums  surrounding  them. 

THE  SUPERIOR  PORTION  OF  THE  MOUTH  (Fig.  5). 

The  osseous  framework,  or  base  upon  which  this  half  of  the  mouth 
is  constructed,  is  composed  of  a  part  of  four  bones — the  two  superior 
maxillary,  or  upper  jaw  bones,  and  the  two  palate  bones  (see  Bones  of 
the  Mouth,  p.  37). 

The  Hard  Palate,  or  Roof  of  the  Mouth  (Figs.  5  and  6). 

This  is  formed  by  the  union  of  the  palatal  processes  of  the  superior 
maxillary  bones  and  the  horizontal  plates  of  the  two  palate  bones  at  the 
median  line.  It  is  limited  in  front  and  laterally  by  the  margins  of  the 
alveolar  process,  or  that  portion  of  the  bone  which  gives  support  to  the 
teeth,  and  ends  posteriorly  in  an  irregular  border,  to  which  is  attached  a 
muscular,  membrane-like  curtain — the  soft  palate.  The  hard  palate  is 
covered  throughout  by  a  thick  and  firm  mucous  membrane,  seldom  so 
highly  colored  as  that  lining  the  lips  and  cheeks.  The  mucous  mem- 
brane is  closely  adherent  to  the  bone  through  its  common  covering,  the 
periosteum.  In  the  center  of  the  hard  palate  is  a  ridge  or  fold  of  mucous 
membrane,  which  follows  the  median  line  from  before  backward;  this 
is  called  the  palatal  or  median  raphe,  and  indicates  the  line  of  union 
formed  during  the  development  of  the  parts.  Anteriorly,  the  raphe  ends 
in  a  small  papilla,  which  marks  the  opening  of  a  canal  in  the  bone — the 
anterior  palatal  canal.  Posteriorly,  the  raphe  usually  diminishes,  in  size, 
but  occasionally  is  well  marked  through  the  whole  extent  of  the  hard 
palate.  Near  the  center  of  the  hard  palate  it  frequently  separates  into 
two  or  more  smaller  ridges,  which  are  proportionately  diminished  in  size, 
and  are  continued  backward  side  by  side.  On  either  side  of  this  central 
ridge,  anteriorly,  the  mucous  membrane  presents  a  number  of  fantas- 
tically arranged  folds,  the  palatal  ruga  (wrinkles).  These  folds  are 
usually  quite  numerous  and  prominent,  but  are  occasionally  but  slightly 
developed.  The  nature  of  these  wrinkles  is  strongly  indicative  of  the  char- 
acter or  temperament  of  the  individual;  thus,  in  the  four  basal  tempera- 
ments they  may  be  divided  as  follows:  in  the  bilious,  heavy  and  strong, 
composed  of  angles  rather  than  curves;  in  the  nervous,  few  in  number, 
close  together,  not  prominent,  and  composed  of  long  curves;  in  the 
sanguine,  quite  numerous,  fairly  prominent,  well  rounded  and  graceful 


THE    INTERIOR    OF    THE    MOUTH 


17 


in  outline;  in  the  lymphatic,  few  in  number,  flat,  widely  separated, 
and  but  little  curved.  Accompanying  these  varying  conditions  in 
the  rugae  will  be  found  a  corresponding  variation  in  the  raphe.  The 
anterior  and  lateral  margins  of  the  mucous  membrane  covering  the 
hard  palate  form  the  palatal  portion  of  the  gingiva  (gums). 


Fig.  5. — The  Superior  Portion  or  Roof  of  the  Mouth. 

In  figure  6  the  hard  palate  is  shown  with  its  mucoperiosteum  re- 
moved. It  will  be  observed  that  the  bony  plates  are  perforated  by  nu- 
merous small  openings  or  foramina,  through  which  the  body  of  the  bone 
receives  its  nourishment,  broken  by  depressions  for  the  accommodation 
of  the  various  mucous  glands,  and  traversed  by  longitudinal  grooves 
which  give  lodgment  to  blood-vessels  and  nerves. 

The  arch  formed  by  the  hard  palate  from  side  to  side  (palatal  arch) 


l8  ANATOMY 

varies  greatly  in  form,  imparting  much  knowledge  in  regard  to  the  tem- 
perament of  the  individual,  and  in  a  measure  controlling  the  quality  of  the 
voice.  Thus,  in  the  sanguine  temperament  the  roof  of  the  mouth  presents 
almost  a  perfect  oval.  In  the  bilious  type  it  is  comparatively  high  and 
flat,  as  it  passes  from  the  base  of  one  alveolar  process  to  the  other,  from 
which  point  it  descends  abruptly  to  the  necks  of  the  teeth.  In  the  ner- 
vous type  the  roof  is  high  and  semi-elliptical  or  parabolic  in  shape,  and 
in  the  lymphatic  it  is  low  and  flat. 

In  the  same  illustration  the  union  or  suture  between  the  four  bones 
may  be  seen  at  the  median  line.  Near  the  anterior  third  of  this  central 
suture  is  the  opening  of  the  incisive  or  anterior  palatal  canal,  the  anterior 
palatal  joramen,  the  location  of  which  has  been  referred  to  in  the  descrip- 
tion of  the  mucous  membrane.  Near  the  posterior  border,  and  situated 
within  the  suture  which  unites  the  superior  maxillary  bones  with  the  palate 
bones  (the  palatomaxillary  suture),  are  two  other  foramina,  the  posterior 
palatal;  and  immediately  behind  these,  and  separated  by  a  thin  ridge  of 
bone,  are  the  accessory  palatal  foramina,  these  being  in  the  tuberosity  of 
the  palate  bones.  (For  further  description  of  these  foramina,  see  Bones 
of  the  Mouth,  p.  37.)  By  the  vessels  and  nerves  which  enter  the  hard 
palate  through  these  various  foramina,  the  mucous  membrane  and 
glands  receive  their  blood-  and  nerve-supply. 

Blood-supply  to  the  Hard  Palate. — This  is  principally  derived 
from  the  posterior  or  descending  palatal  branch  of  the  internal  maxillary 
or  deep  facial  artery,  which  passes  downward  in  the  posterior  palatal 
canal  and  emerges  through  the  posterior  palatal  foramen.  Immedi- 
ately on  reaching  the  palate  it  divides  into  an  anterior  and  a  posterior 
branch,  the  former  passing  forward  in  a  groove  provided  for  it  to  the  an- 
terior palatal  foramen,  where  it  anastomoses  with  the  nasopalatal  artery. 
The  groove  in  which  the  artery  lies  in  its  passage  forward  is  usually  at 
the  base  of  the  alveolar  process,  and  in  some  instances  is  converted  into 
a  canal  for  a  part  of  its  length.  The  posterior  branches  pass  backward 
and  downward  to  supply  the  soft  palate.  In  connection  with  supplying 
the  hard  palate  proper,  this  artery  carries  blood  to  the  palatal  alveolar 
walls,  to  the  mucous  glands,  the  mucous  membrane,  and  the  gums. 

Course  of  the  Blood  from  the  Heart  to  the  Hard  Palate.— 
From  the  heart  to  the  aorta,  to  the  common  carotid,  to  the  external 
carotid,  to  the  internal  maxillary,  to  the  posterior  or  descending  palatal 
branch  of  the  latter.  From  the  hard  palate  the  blood  is  returned  to  the 
heart  by  the  superior  palatal  and  inferior  or  descending  palatal  veins,  the 
former  following  the  course  of  the  superior  palatal   artery,  while  the 


THE    INTERIOR    OF    THE    MOUTH  19 

latter  originates  at  a  point  near  the  junction  of  the  hard  and  soft  palates, 
passes  downward,  and  joins  the  facial  vein  below  the  body  of  the  inferior 
maxilla. 

Nerves  of  the  Hard  Palate. — The  nerves  of  the  hard  palate  are 
the  anterior  or  large  palatal  and  branches  from  the  nasopalatal,  both  of 
which  are  branches  of  the  sphenopalatal  (Meckel's)  ganglion.  The  an- 
terior palatal  nerve  arises  from  the  inferior  angle  of  the  ganglion,  passes 


Fig.  6. — The  Hard  Palate,  or  Roof  of  the  Mouth,  with  its  Membranous 
Covering  Removed. 

downward,  accompanied  by  the  descending  palatal  artery,  through  the 
posterior  palatal  canal,  from  which  it  emerges  at  the  posterior  palatal 
foramen.  From  this  point  it  passes  forward  in  a  groove  of  the  hard 
palate,  and  joins  the  nasopalatal  nerve  as  it  emerges  from  the  anterior 
palatal  foramen.  Accompanying  this  nerve  in  its  course  through  the 
posterior  palatal  canal  are  other  branches  of  the  sphenopalatal  ganglion, 
which  pass  to  the  soft  palate,  and  will  be  described  in  that  connection. 
The  nerves  of  the  hard  palate  are  all  sensory,  and  filaments  are  dis- 
tributed to  the  mucous  membrane  and  glands  and  to  the  palatal  portion 
of  the  gums. 

THE  SOFT  PALATE  (Figs.  6  and  7). 

The  soft  palate  is  attached  to  the  posterior  border  of  the  hard  palate, 
from  which  it  is  continued  as  a  backward  prolongation  of  its  soft  tissue. 


20  ANATOMY 

Hanging  downward,  with  its  free  borders  inclining  backward,  it  may  be 
considered  as  forming  a  part  of  the  posterior  boundary  of  the  mouth.  It 
partially  separates  the  mouth  from  the  nasal  cavity  and  from  the  pharynx. 
It  is  attached  laterally  to  the  walls  of  the  pharynx,  while  its  lower  border 
is  free.  The  substance  of  the  soft  palate  is  composed  of  a  number  of  thin, 
but  dense,  muscular  fibers,  blood-vessels,  nerves,  and  mucous  glands,  the 
latter  being  similar  to  those  of  the  hard  palate.  The  anterior  surface 
of  this  muscular  curtain  is  concave,  directed  forward  and  downward, 
and  is  traversed  by  a  mecli an  "raphe.  The  posterior  surface  is  convex, 
directed  backward  and  upward,  and  is  continuous  with  the  nasal  cavity. 
Suspended  from  the  center  of  its  free  border  is  a  small  rounded  or  conic 
membranous  appendix,  the  uvula,  and  passing  outward  from  the  base 
of  this  at  each  side  are  two  curved  muscular  folds  which  extend  outward 
and  downward,  and  are  known  as  the  pillars  of  the  fauces.  From  the 
position  which  these  folds  occupy  they  are  divided  into  the  anterior  and 
the  posterior  pillars  of  the  fauces.  The  anterior  pillar  is  formed  from 
muscular  fibers  which  extend  from  the  soft  palate  to  the  side  and  base  of 
the  tongue  (palatoglossus  muscle),  and  is  somewhat  prominent  as  it  passes 
downward,  outward,  and  forward.  The  posterior  pillar  approaches 
more  closely  to  its  fellow  of  the  opposite  side  than  does  the  anterior. 
It  is  formed  of  muscular  fibers  which  extend  from  the  soft  palate  above 
to  the  pharynx  below  (palatopharyngeus  muscle).  It  is  somewhat  con- 
cave in  its  downward  and  backward  course,  and  while  closely  united  to 
the  anterior  pillar  above,  is  separated  from  it  below,  leaving, a  triangular 
interval  or  niche  in  which  is  lodged  a  small,  almond-shaped  body,  the 
tonsil,  the  space  being  known  as  the  tonsillar  recess.  The  intervening 
space — bounded  by  the  margins  of  the  soft  palate  above,  the  root  of  the 
tongue  below,  and  the  pillars  laterally — is  called  the  isthmus  of  the  fauces, 
which  establishes  the  communication  between  the  mouth  and  the  pharynx. 
The  free  margins  of  the  soft  palate,  assisted  by  the  pillars  of  the  fauces, 
mark  the  posterior  boundary  of  the  mouth.  The  entire  surface  of  the  soft 
palate  and  its  prolongations,  the  pillars  of  the  fauces,  is  covered  with 
mucous  membrane,  being  continuous  with  that  of  the  mouth  on  its  an- 
terior surface,  and  with  that  of  the  nasal  cavity  on  its  posterior  surface. 
Muscles  of  the  Soft  Palate. 

On  each  side  the  muscles  of  the  soft  palate  are  the  palatoglossus, 
palatopharyngeus,  levator  palati,  and  tensor  palati,  together  with  the 
azygos  uvula. 

Palatoglossus. — A  small  fasciculus  of  fibers,  somewhat  cylindric 
in  form,  expanding  at  either  end  into  a  thin  sheet.     It  is  named  from  its 


THE    INTERIOR    OF    THE    MOUTH 


21 


attachment  to  the  soft  palate  and  to  the  tongue.  It  is  the  prominence  of 
this  muscle,  together  with  its  covering  of  mucous  membrane,  that  forms 
the  anterior  pillar  of  the  fauces. 


Pharyngeal  _ 
aponeurosis 

Levator  veli 
palatini 


Tensor  veli 
palatini 


AUDITORY  OR 

EUSTACHIAN  TUBE 


M.  uvulae 
Hamular  process 


Fharyngo- 
palatinus 


Pharyngo-palatinus 


Constrictor  pharyngis 
superior 


Thyreoid  cartilage 


Cricoid  cartilage 


Crico-arytaenoideus 
posterior 


FlG.  y  _view  of  Muscles  of  Soft  Palate,  as  seen  from  within  the  Pharynx. 
(Morris  Modified  from  Bourgery.) 

Origin.— By  a  thin  muscular  sheet  from  the  under  surface  of  the 
aponeurosis  of  the  soft  palate  near  the  median  line,  its  fibers  uniting 


22  ANATOMY 

with  those  of  the  opposite  side.  It  passes  downward  in  front  of  the  ton- 
sil and  against  the  pharyngeal  wall. 

Insertion. — Into  the  side  and  base  of  the  tongue. 

Relations. — Superficially,  it  is  covered  by  the  mucous  membrane 
of  the  soft  palate  and  tongue;  deeply,  in  contact  with  the  aponeurosis 
of  the  soft  palate,  the  superior  constrictor  muscle  of  the  pharynx,  and 
one  of  the  muscles  of  the  tongue — the  hyoglossus. 

Action. — The  lateral  walls  of  the  soft  palate  are  drawn  down,  and 
the  sides  of  the  tongue  are  drawn  upward  and  slightly  backward.  Acting 
in  conjunction  with  the  palatopharyngeus,  the  opening  of  the  fauces  is 
constricted. 

Palatopharyngeus. — This  muscle — also  named  from  its  attach- 
ments— is  broad  above,  where  it  forms  the  greater  part  of  the  lower 
half  of  the  soft  palate.  Near  the  median  line  a  few  of  its  fibers  blend 
with  those  of  its  fellow  of  the  opposite  side. 

Origin. — By  two  heads  from  a  point  near  the  raphe  or  median  line 
of  the  soft  palate,  passing  downward  and  slightly  backward,  forming, 
with  its  covering  of  mucous  membrane,  the  posterior  pillar  of  the 
fauces. 

Insertion.— Into  the  posterior  border  of  the  thyroid  cartilage,  and  to 
the  inner  surface  of  the  lower  part  of  the  pharynx. 

Relations. — In  the  soft  palate,  superficially,  with  the  mucous  mem- 
brane, both  anteriorly  and  posteriorly;  above,  with  the  levator  palati; 
and  beneath,  by  the  mucous  glands.  In  the  posterior  pillar  it  is  sur- 
rounded with  mucous  membrane,  and  in  the  pharynx  by  the  constrictor 
muscles  of  the  pharynx  and  the  mucous  membrane. 

Action. — To  constrict  the  opening  of  the  fauces,  by  bringing  together 
the  posterior  pillars,  thus  depressing  the  soft  palate  and  elevating  the 
pharynx.  It  controls  the  position  of  the  soft  palate  during  respiration, 
and  elevates  the  pharynx  during  deglutition. 

Levator  Palati. — This  is  a  moderately  thick  muscle,  and  derives 
its  name  from  its  action  upon  the  soft  palate. 

Origin. — By  a  short  tendon  from  the  under  surface  of  the  petrous 
portion  of  the  temporal  bone,  and  from  the  posterior  and  inferior  aspect 
of  the  cartilage  of  the  Eustachian  tube. 

Insertion. — After  passing  downward  by  the  side  of  the  posterior  nares 
it  is  inserted  into  the  median  line  of  the  soft  palate,  where  its  fibers 
unite  with  those  of  its  fellow  of  the  opposite  side. 

Relations. — Externally,  with  the  tensor  palati  and  superior  con. 
strictor  muscles;  internally  and  posteriorly,  with  the  mucous  membrane- 


THE    INTERIOR    OF    THE    MOUTH  23 

Action. — To  raise  the  soft  palate,  bringing  it  against  the  posterior 
wall  of  the  pharynx. 

Tensor  Palati. — This  is  a  slender  and  flattened  muscular  sheet, 
and  receives  its  name  from  its  action  upon  the  soft  palate. 

Origin. — From  the  scaphoid  fossa  at  the  base  of  the  internal  ptery- 
goid plate  and  the  spinous  process  of  the  sphenoid  bone;  also  from  the 
outer  side  of  the  anterior  aspect  of  the  Eustachian  tube. 

Insertion. — After  descending  between  the  internal  pterygoid  muscle 
and  the  internal  pterygoid  plate,  and  winding  around  the  hamular  process 
of  the  latter,  it  is  inserted  into  the  transverse  ridge  on  the  horizontal 
portion  of  the  palate  bone,  and  at  the  median  line  of  the  soft  palate 
where  it  is  continuous  with  the  aponeurosis  of  the  opposite  side. 

Action. — To  tighten  or  spread  the  soft  palate  laterally,  forming  a 
septum  between  the  posterior  nares  and  the  pharynx.  It  also  opens  the 
Eustachian  tube  during  deglutition. 

The  azygos  uvulae  (so  named  because  it  was  at  one  time  supposed 
to  be  a  single  muscle)  is  composed  of  a  pair  of  small  muscles  which  orig- 
inate from  the  aponeurosis  of  the  soft  palate  and  the  nasal  spine  of  the 
palate  bone.  They  pass  downward  and  form  or  are  inserted  into  the 
uvula. 

Relations. — Anteriorly,  with  the  levator  palati,  palatoglossi,  and  a 
part  of  the  palatopharyngei;  posteriorly,  to  the  mucous  membrane. 

Action. — To  shorten  or  draw  up  the  uvula. 
Blood-supply  to  the  Soft  Palate. 

The  poslerior  or  descending  palatal  branch  of  the  deep  facial  artery, 
after  emerging  from  the  posterior  palatal  canal,  sends  its  posterior  divi- 
sion backward  and  downward  to  the  soft  palate,  in  the  substance  of  which 
they  anastomose  with  the  ascending  palatal  artery.  After  passing  over 
the  superior  border  of  the  pharynx,  the  ascending  pharyngeal  artery  sends 
off  branches  which  are  distributed  to  the  soft  palate.  A  few  branches  of 
the  superior  palatal  branch  of  the  internal  maxillary  and  a  few  twigs 
from  the  lingual  artery  also  convey  blood  to  the  parts. 

Course  of  the  Blood  from  Heart  to  the  Soft  Palate. — From  the 
heart  to  the  aorta,  to  the  common  carotid,  to  the  external  carotid,  to  the 
facial  or  lingual,  to  the  various  branches  named  above,  to  the  soft  palate. 
The  return  of  the  blood  to  the  heart  is  principally  through  the  superior 
and  inferior  or  descending  palatal  veins,  both  of  which  closely  follow  the 
course  of  the  arteries  of  the  same  name. 

Nerves  of  the  Soft  Palate. — The  small,  or  posterior,  palatal,  the 
external  palatal   (both    of  which   are   branches   of   Meckel's   ganglion), 


24 


ANATOMY 


branches  of  the  glossopharyngeal  nerve,  and  the  following  nerves  which 
supply  the  various  muscles:  filaments  from  the  pharyngeal  plexus  to  the 
palatoglossus  and  palatopharyngeus,  branches  of  the  Vidian  to  the  leva- 
tor palati  and  azygos  uvulae,  and  from  the  mandibular  division  of  the  fifth 
nerve  to  the  tensor  palati.* 

THE  INFERIOR  PORTION  OR  FLOOR  OF 
THE  MOUTH  (Fig.  8). 

This  half  of  the  cavity  of  the  mouth  contains  the  tongue  and  its 
attached  muscles,  the  sixteen  lower  teeth  firmly  implanted  in  the  jaw,  and 


Fig.  8. — The  Inferior  Portion  or  Floor  of  the  Mouth. 

the  gums  covering  the  alveolar  walls.  The  base  or  osseous  framework 
upon  which  this  portion  of  the  mouth  is  constructed  is  principally  made 
up  of  a  single  bone,  the  inferior  maxillary,  mandible,  or  lower  jaw  bone 

*  A  description  of  the  upper  teeth  will  be  found  in  another  chapter. 


THE    INTERIOR    OF    THE    MOUTH 


25 


(see  Bones  of  the  Mouth,  p.  37).  The  hyoid  bone,  situated  between  the 
angles  of  the  mandible  in  the  upper  part  of  the  neck,  and  at  the  base  of 
the  tongue,  giving  attachment  to  many  of  the  muscles  about  the  floor  of 
the  mouth,  may  also  be  considered  in  this  connection.  The  floor  of  the 
mouth  is  bounded  anteriorly  by  the  lower  lip,  laterally  by  the  cheeks, 
and  below  by  the  muscles  attached  to  the  external  and  internal  oblique 
lines  of  the  mandible. 


THE  TONGUE  {Lingua)   (Fig.  9). 
The  tongue  is  a  freely  movable,  highly  sensitive,  muscular  organ.     It 


True  Vocal  Cord        Posterior  Wall  of  the  Pharynx       Corniculum  Laryngis 


False  Vocal  Cord 


Tonsil 


Adenoid  Tissue 
at  Base  of  Tongue 


Foramen  Caecurr. 


Epiglottis 


Median  Glosso- 
epiglottidean  Fold 


Fungiform  Papillae     Circumvallate  Papillae 
Fig.  9. — Superior  Aperture  of  Larynx.     (Deaver.) 


assists  in  the  function  of  speech,  participating,  also,  in  the  special  sense 


26 


ANATOMY 


of  taste,  and  in  mastication  and  deglutition.  The  organ  is  attached 
posteriorly  to  a  U-shaped  bone,  the  hyoid  bone,  which  of  itself  is  movable, 
and  is  placed  in  the  neck  between  the  angles  of  the  mandible  and  the 
thyroid  cartilage. 

The  tongue  is  suspended  and  kept  in  its  position  in  the  mouth  by 
numerous  muscles,  some  of  which  are  attached  to  the  base  of  the  skull, 
and  others  to  the  mandible  and  hyoid  bone. 

The  size  of  the  tongue  bears  little  or  no  relation  to  the  size  of  the  indi- 
vidual, but  is  proportioned  to  the  capacity  of  the  alveolar  arch,  which  space 

it  completely  fills  when  at  rest.  The 
shape  of  the  tongue  is  influenced  by 
the  shape  of  the  alveolar  arch;  thus, 
when  the  arch  is  contracted,  narrow, 
and  pointed,  the  margins  of  the 
tongue,  when  at  rest,  will  assume 
that  form  (a,  Fig.  10) ;  but  when  the 
arch  is  broad  and  rounded  anteriorly, 
the  margins  of  the  tongue  will  also 
be  broad  and  rounded  (b,  Fig.  10). 

The  substance  of  the  tongue  is 
chiefly  composed  of  muscular  fibers, 
which  are  arranged  in  a  complicated 
manner,  crossing  one  another  at  various  angles,  thus  making  the 
movements  of  the  organ  exceedingly  varied  and  extensive.  Fibrous, 
areolar,  and  fatty  and  glandular  tissue  enter  into  its  structure,  and  it  is 
freely  supplied  with  blood-vessels  and  nerves.  Its  free  surface  is  covered 
by  a  specialized  mucous  membrane,  and  over  its  entire  surface  are 
numerous  mucous  follicles  and  glands. 

Before  continuing  the  description  of  the  tongue,  it  will  be  necessary  to 
name  its  parts.  The  upper  surface,  or  that  facing  the  roof  of  the  mouth, 
is  the  dorsum  {dor sum  Ungues);  those  portions  directed  toward  the  cheeks 
are  known  as  the  margins  of  the  tongue;  the  thin  narrow  portion  directed 
forward  and  against  the  inner  surface  of  the  lower  front  teeth,  is  the 
apex  or  tip  (apex  lingua).  That  portion  between  the  frenum  and  extend- 
ing back  to  the  pillars  of  the  fauces  is  the  base,  while  that  part  of  the  dor- 
sum immediately  posterior  to  the  tip  is  the  post-tip,  that  region  which 
lies  between  the  post-tip  and  the  base  being  the  prebase.  The  dorsum, 
sides,  and  tip  are  free,  while  the  base  is  attached  by  muscles  to  the  man- 
dible and  hyoid  bone. 

From  the  base  to  the  epiglottis  is  a  fold  which  serves  to  limit  the 


Fig.  io. 


THE    INTERIOR    OF    THE    MOUTH  27 

movement  of  the  latter  organ,  and  to  the  sides  of  the  base  the  pillars  of 
the  fauces  are  attached.  Under  the  anterior  free  extremity  in  the  median 
line  is  a  fibrous  muscular  lamina  or  ligament,  the frenum  (frenum  linguae), 
which  connects  this  part  of  the  organ  with  the  lower  jaw  and  marks  the 
anterior  border  of  the  base  of  the  tongue.  The  tongue  is  divided  through 
its  anterior  two-thirds  by  a  slight  longitudinal  furrow,  the  median  raphe, 
which  ends  posteriorly  near  a  small  foramen,  not  constant  in  the  adult 
tongue,  but  plainly  observed  in  the  fetus  or  infant,  the  foramen  coecum. 
This  foramen  represents  the  upper  termination  of  the  thyreoglossus  duct. 

Papillae  of  the  Tongue. — Over  the  anterior  two-thirds  of  the  dor- 
sum and  the  sides  and  tip  of  the  tongue  are  a  number  of  small,  soft, 
conic  eminences,  which  are  known  as  the  papillae  of  the  tongue.  These 
are  most  numerous  over  the  anterior  part  of  the  dorsum,  and  at  the  back 
they  are  covered  and  partly  hidden  by  an  epithelial  coating.  In  general, 
the  papillae  are  quite  similar  to  those  of  the  integument,  not  being  com- 
pound organs  in  their  vascular  and  nervous  supply.  In  consequence  of 
their  variation  in  form  and  arrangement  the  papillae  are  variously 
named.  The  largest  papillae,  being  arranged  like  the  letter  V,  are  called 
the  circumvallate  or  falciform;  those  of  medium  size,  the ■fungiform,  are 
so  named  from  their  resemblance  to  a  young  mushroom;  and  the  smallest 
and  most  numerous  are  known  as  the  conic  or  filiform  papillae.  Each 
papilla  presents  a  broad,  free  end,  and  is  attached  by  a  constricted  base, 
which  rests  in  a  small,  cup-like  concavity,  about  the  margins  of  which  is  a 
well-formed  circular  rim.  Beneath  the  thick  epithelium  of  these  parts 
are  numerous  secondary  papillae,  and  about  the  base  of  each  papilla 
are  the  openings  of  one  or  more  glands. 

The  circumvallate  or  falciform  papillae  (Fig.  9)  form  a  V-shaped  line 
at  the  posterior  boundary  of  the  dorsum.  They  are  few  in  number 
(varying  from  six  to  twelve),  but  are  largest  in  size,  not  infrequently  meas- 
uring 1/4  of  an  inch  in  diameter.  These  papillae  are  generally  regarded 
as  being  gustatory,  or  directly  interested  in  the  sense  of  taste.  Each 
papilla  is  capped  with  a  small  secondary  papilla. 

The  fungiform  papillae  (Fig.  9),  of  medium  size,  varying  from  1/20 
to  1/50  of  an  inch  in  diameter,  are  scattered  over  the  dorsum,  sides,  and 
tip  of  the  tongue  at  irregular  intervals,  and  are  much  more  highly  colored 
than  the  smaller  papillae  which  surround  them.  They  vary  greatly  in 
number,  and  being  principally  gustatory,  account  in  a  great  measure  for 
the  diversity  in  the  acuteness  of  the  sense  of  taste  in  different  individuals. 
These  papillae,  like  the  circumvallate,  are  capped  with  smaller  secondary 
papillae. 


28  ANATOMY 

The  conic  ox  filiform  papillae  (Fig.  9)  are  the  smallest  and  most  numer- 
ous, and  are  thickly  scattered  over  the  entire  surface  of  the  dorsum  in 
front  of  the  circumvallate  as  well  as  over  the  sides  and  tip  of  the  tongue. 
They  are  placed  closely  together,  and  with  such  regularity  that  they 
fairly  ridge  the  tongue  with  delicate  lines,  which  run  parallel  with  the 
circumvallate  in  that  region,  but  as  the  tip  is  approached  they  become 
transversely  inclined.  These  papillae  are  generally  regarded  as  being 
tactile  or  directly  interested  in  the  sense  of  touch,  and  are  concerned  in 
directing  the  movements  of  the  food  during  mastication.  They  also 
possess  secondary  papillae  upon  their  surfaces. 

Immediately  posterior  to  the  circumvallate  papillae  are  two  shallow 
grooves  which  follow  the  V-shaped  line  of  the  papillae  and  unite  at  the 
foramen  caecum.  These  grooves  serve  to  indicate  the  line  of  junction 
between  the  anterior  and  posterior  portions  of  the  tongue.  The  latter  not 
being  within  the  cavity  of  the  mouth  will  not  be  described. 

Muscles  of  the  Tongue  (Fig.  n). 

The  muscles  of  the  tongue  are  both  extrinsic-outward  or  external,  and 
intrinsic-inherent,  inward,  or  special. 

The  extrinsic  muscles  include  those  which  have  their  origin  from 
the  base  of  the  skull,  the  hyoid  bone,  or  the  mandible,  and  are  the  hyo- 
glossus,  geniohyoglossus,  styloglossus,  palatoglossus,  and  a  few  fibers  of 
the  superior  constrictor  of  the  pharynx.  The  intrinsic  muscles  which 
make  up  the  bulk  of  the  tongue  are  two  in  number,  the  superior  lingualis 
and  inferior  lingualis. 

Hyoglossus.— As  its  name  implies,  this  muscle  extends  from  the 
hyoid  bone  to  the  tongue.  Its  fibers  are  so  arranged  that  they  form  a 
thin  square  sheet. 

Origin. — It  arises  from  the  whole  length  of  the  upper  border  of  the 
great  cornu,  from  the  body,  and  by  a  few  fibers  from  the  lesser  cornu  of 
the  hyoid  bone.  At  their  point  of  origin  the  fibers  are  in  the  form  of  a 
thin  sheet,  and  ascend  toward  the  tongue  almost  parallel  to  one  another, 
but  before  reaching  the  tongue  the  anterior  fibers  pass  slightly  forward, 
and  at  the  upper  margin  of  the  side  of  the  tongue  bend  inward  and  join 
the  fibers  of  the  superior  lingualis.  In  their  distribution  to  this  part  of 
the  tongue  they  form  a  kind  of  submucous  covering  to  the  organ. 

Insertion. — Into  the  posterior  half  of  the  side  of  the  tongue,  between 
the  styloglossus  and  superior  lingualis  muscles. 

Relations. — Externally,  with  the  digastricus,  styloglossus,  stylohyoid, 
and  mylohyoid  muscles,  the  lingual  and  hypoglossal  nerves,  Wharton's 


THE    INTERIOR    OF    THE    MOUTH 


29 


duct,  and  the  sublingual  gland.  Internally,  with  the  lingualis,  geniohyo- 
glossus,  and  middle  constrictor  of  the  pharynx  muscles,  the  lingual  artery, 
and  the  glossopharyngeal  nerve. 

Action. — To  extend  the  tongue  and  to  draw  it  backward,  also  to 


Stylo-glossus 


DORSUM  OF  TONGUE 


Genio-hyo-glossus 
Genio-hyoid 


GREATER  COPNU  OF  HYOID  BONE 


r~    STYLOID  PROCESS 
Stylo-hyoid 


POSTERIOR  PORTION 
OF  TONGUE 


Stylo-pharyngeus 


Hyo-glossuB 

Thyro-hyoid 
ligament 

CARTILAGO  TRITICEA 


Thyro-hyoid 
membrane 


THYROID  CARTILAGE 


Median   portion   of 
erieo-thyroid 
membrane 

CRICOID  CARTILAGE 
FIRST  RING  OF  TRACHEA 


p'>'r?i" """,frfM! 

Fig.  11. — Side  View  of  the  Tongue,  with  its  Muscles.     (Mrrois.) 


draw  downward  the  sides  of  the  tongue,  making  its  dorsum  more  convex 
transversely. 

Geniohyoglossus  (Fig.  11). — This  muscle  also  receives  its  name 
from  its  three  points  of  attachment,  the  chin  internally,  the  hyoid  bone, 
and  the  tongue.  It  is  a  triangular-shaped  muscle,  narrow  and  pointed 
at  its  attachment  to  the  mandible,  and  broad  and  fan-shaped  on  approach- 
ing the  tongue.     Being  near  the  median  line,  it  is  separated  from  its 


3° 


ANATOMY 


fellow  of  the  opposite  side  by  a  thin  layer  of  connective  tissue,  the  septum 
of  the  tongue. 

Origin. — It  arises  by  a  short  tendon  from  the  upper  genial  tubercle 
of  the  lower  jaw,  from  which  point  its  fleshy  fibers  diverge  fan-like  to  its 
extensive  insertion. 

Insertion. — To  the  whole  length  of  the  tongue  from  base  to  apex 


Vertical  and  Trans- 

Mucous    Submucous  Superior  Linguaiis        verse  Muscular  Fibers 

Membrane    l  issue  Muscle 


Inferior  Linguaiis  Muscle 
Vena  Comes  ^ 

Ranine  Arteries     Vena  Comes 
Initrnsic  Muscular  Fibers 

Fig.   12. — Transverse  Section  of  One-half  of  Tongue.     (Deaver.) 


immediately  external  to  the  median  line,  into  the  body  of  the  hyoid  bone, 
and  by  a  few  fibers  into  the  side  of  the  pharynx. 

Relations. — By  its  inner  surface,  with  the  septum  of  the  tongue  and 
its  fellow  of  the  opposite  side;  by  its  outer  surface,  with  the  hyoglossus, 
mylohyoides,  styloglossus,  and  linguaiis  muscles,  sublingual  gland,  lin- 
gual artery,  and  hypoglossal  nerve.  Superiorly,  with  the  mucous  mem- 
brane of  the  floor  of  the  mouth;  inferiorly,  with  the  geniohyoid  muscle. 


THE    INTERIOR    OF    THE    MOUTH 


31 


Action. — Its  anterior  fibers  assist  in  drawing  back  the  tip  of  the 
tongue,  its  posterior  fibers  throwing  forward  and  protruding  the  tongue. 
This  muscle  also  depresses  the  center  of  the  dorsum  longitudinally, 
making  it  concave  transversely,  and  some  of  the  lower  fibers  which  are 
attached  to  the  hyoid  bone  elevate  the  bone  and  assist  in  raising  the  tongue. 

Styloglossus. — Also  named  from  its  attachment,  is  a  long  fan-shaped 
muscle,  somewhat  compressed  laterally. 


FR/ENUM  L1NGU/E 


Iiingualis  inferior 


Hyo-glossus 


Genio-hyoid 


Mylo-hyoid,  reflected 


Sterno-hyoid 


-   Iiingualis  inferior 


Genie— hyo-glossus 


Stylo-glossus 
Hyo-glossus 
BODY  OF  HYOID  BONE 
Genio-hyoid 


THYROID  CARTILAGE 


Fig.   13. — Under  Surface  of  the  Tongue  with  Muscles.     (Morris.) 


Origin. — From  its  point  of  origin  at  the  tip  of  the  styloid  process  of 
the  temporal  bone,  and  from  a  portion  of  the  stylomaxillary  ligament,  it 
passes  with  a  long  curve,  forward,  slightly  downward,  then  upward  and 
inward  to  its  place  of  insertion  at  the  side  of  the  tongue. 

Insertion. — Upon  reaching  the  side  of  the  tongue  it  divides  into  two 
portions,  the  fibers  of  one  portion  passing  transversely  inward,  while  the 
others  pass  longitudinally  along  the  side  of  the  tongue. 

Relations. — Externally,  with  the  internal  pterygoid  muscle,  parotid 


32  ANATOMY 

and  sublingual  glands,  lineal  nerve,  and  the  mucous  membrane  of  the 
mouth;  internally,  with  the  superior  constrictor  and  hyoglossus  muscles, 
and  with  the  tonsil. 

Action. — To  draw  the  tongue  backward  and  to  produce  a  transverse 
concavity  to  its  upper  surface  by  elevating  its  sides. 

Superior  Lingualis. — This  is  one  of  the  intrinsic  muscles,  and  is 
situated  immediately  beneath  the  mucous  membrane,  extending  from  the 
base  to  the  tip  of  the  organ. 

Inferior  Lingualis  (Fig.  13). — This  muscle  is  placed  near  the  under 
surface  of  the  tongue,  and  is  composed  of  two  bands  which  extend  from 
base  to  apex,  some  of  its  fibers  being  attached  posteriorly  to  the  hyoid 
bone,  and  in  passing  forward  are  placed  between  the  hyoglossus  and  genio- 
hyoglossus.     Anteriorly,  its  fibers  blend  with  those  of  the  styloglossus. 

Many  of  the  fibers  of  this  muscle  run  transversely  and  are  placed 
between  the  two  former  intrinsic  muscles.  These,  together  with  some 
fatty  tissue  compose  the  greater  part  of  the  substance  of  the  tongue. 
The  fibers  are  attached  at  the  median  line  to  the  fibrocartilaginous  sep- 
tum of  the  tongue,  and  laterally  to  the  mucous  membrane.  In  connection 
with  the  transverse  fibers  there  are  a  few  placed  vertically,  which  pass  by 
long  curves  from  the  dorsum  to  the  under  surface  of  the  tongue. 

Blood-vessels  of  the  Tongue  (Fig.  14). 

This  organ  receives  its  blood  principally  through  the  lingual,  facial, 
and  ascending  pharyngeal  arteries.  The  lingual  artery  arises  from  the 
front  of  the  external  carotid  near  the  facial,  and  often  as  a  common  trunk 
with  it.  From  its  point  of  origin  to  the  tongue  it  is  divided  into  three 
portions,  the  first  or  oblique,  the  second  or  horizontal,  and  the  third  or 
ascending,  and  it  is  this  latter  portion  which  directly  supplies  the  tongue. 
Ascending  tortuously  beneath  the  hyoglossus  muscle,  it  reaches  the  under 
surface  of  the  tongue,  and,  lying  between  the  lingualis  and  hyoglossus 
muscles,  it  is  continued  to  the  under  surface  of  the  tip  of  the  tongue,  at 
which  point  it  is  called  the  ranine  artery.  At  a  point  about  corresponding 
with  the  posterior  margin  of  the  hyoglossus  muscle  a  branch  is  given  off 
(the  dorsalis  linguce),  which  passes  almost  directly  upward,  and,  after 
dividing  into  two  or  more  small  branches,  supplies  the  back  part  of  the 
dorsum  of  the  tongue  and  the  mucous  membrane  about  the  circumvallate 
papillae.  At  the  anterior  border  of  the  hyoglossus  muscle  another  branch 
is  given  off  (the  sublingual  artery)  supplying  the  anterior  muscular  struc- 
ture of  the  floor  of  the  mouth.  The  facial  artery  by  one  of  its  muscular 
branches  supplies  the  styloglossus  muscle. 


THE    INTERIOR    OF    THE    MOUTH 


33 


Course  of  the  Blood  from  the  Heart  to  the  Tongue. — From  the 
heart  to  the  aorta,  to  the  common  carotid,  to  the  external  carotid,  to  the 
lingual  artery  and  its  smaller  branches  to  the  tongue.  From  the  tongue 
the  blood  is  returned  to  the  heart  principally  through  the  lingual  vein, 
which  begins  at  the  ranine  vein  beneath  the  tip  of  the  tongue,  passes  back- 
ward under  cover  of  the  mucous  membrane,  following  the  course  of  the 
lingual  artery  until  the  hyoglossus  muscle  is  reached,  beyond  which  point 


Posterior  br  of 
descending  palatine  A 

Palatine  br.  of 
a-scendm  qph  aiyngeal  A 

Ascending  phary -ngealA*- 

Ascendinq  palatine  br 
J  'of facial  A 
Tonsillar    br. 
of  facial  A. 
Stylo  pharynqeus  M- 

Facial  A. 

Middle  conslriclorM '.- 
Dors  a  lis  linguae  A 

Lingual  A  — 
External  carotidA 


-Descending  pal  a  line  A . 

-A  n  terior  br.  of  descending  pala  tine  A . 
,Stylo-glossusM. 

,Palato  -glossusM. 

,  Tonsillar  br  dors  a  I  is  linguae  A 


Superior  Jhyroid 

Infra-  hyoid   br  of '  s^p.thyroidT^j)^qastllc }f 
Supra-  hvoid  br  "■>    <■' 

of  lingual  A 


rtery  of  f men  urn 
Subnifntiil  A. 
Cenio-hyuid  M 
Genio  -hvo  -glossus  M. 
Stylo  -hyoid  M.     ^Sublingual  A . 


Fig.   14. — Arteries  of  Tongue  and  Tonsil.     (Deaver.) 

the  fibers  of  the  muscle  separate  the  artery  from  the  vein.     After  receiving 
the  sublingual  and  dorsalis  linguae  veins,  the  course  of  which  corresponds 
to  the  arteries  of  the  same  name,  the  vein  passes  backward  and  down- 
ward and  empties  into  the  internal  jugular. 
Nerves  of  the  Tongue. 

The  mandibular  division  of  the  fifth  nerve  by  its  lingual  branch  sup- 
plies the  papillae  of  the  anterior  portion  and  sides  of  the  tongue,  while  the 
lingual  branch  of  the  glossopharyngeal  supplies  the  circumvallate  papillae, 
the  base,  and  posterior  sides.  A  few  branches  of  the  superior  laryngeal 
are  distributed  to  the  back  part  of  the  root  of  the  organ.  The  motor 
nerve  of  the  tongue  is  the  hypoglossal  or  ninth,  supplying  both  the  extrinsic 
and  intrinsic  muscles.* 

*  A  description  of  the  lower  teeth  will  be  found  in  another  chapter. 
3 


CHAPTER  II. 

Muscular  Tissues  of  the  Mouth;  of  the  Lips;  of   the  Cheeks;  of 
the  Soft  Palate ;  of  the  Tongue. 

MUSCULAR  TISSUES  OF  THE  MOUTH. 

Muscular  Tissues  of  the  Lips. — The  minute  bundles  forming  the 
fasciculi  of  the  oral  sphincter  muscle — the  orbicularis  oris — are  distributed 
between  the  submucosa  of  the  mucomembranous  portion  and  the  sub- 
cutaneous tissue  of  the  cutaneous  portion  of  the  lips.  The  muscular 
fibers  radiate  in  three  principal  directions  upon  either  side  of  the  median 
line:  from  the  angle  of  the  mouth  toward  the  median  line,  and  from  the 
fleshy  slips  of  the  maxilla  and  mandible — the  musculi  incisivi.  As  the 
fibers  from  the  angle  of  the  mouth  pass  to  the  substance  of  the  lip,  they 
are  arranged  in  a  laminated  manner.  When  the  median  line  is  reached, 
one  set  of  fibers  terminates  somewhat  abruptly  in  the  subcutaneous  tissue, 
another  set  is  continued  beyond  the  median  line  and  attached  to  the  cutis 
of  the  opposite  side,  while  a  third  set,  without  crossing  the  median  line, 
is  attached  to  the  incisive  fossae  of  the  maxilla  and  mandible.  The  numer- 
ous muscular  fibers  of  the  internal  labial  or  mucomembranous  portion, 
and  the  external,  facial,  or  cuticular  portion,  penetrate  the  parts  and 
terminate  in  close  proximity  to  the  epithelium  or  to  the  base  of  the  papillae. 
Delicate,  hair-like  fibers  which  are  continuous  with  the  sarcolemma 
slightly  penetrate  the  cutis  and  membrana  propria.  A  few  of  the  fibers, 
which  may  be  classed  with  the  terminals  of  the  outrunning  muscles 
from  the  lips,  are  arranged  in  a  number  of  fasciculi  in  the  subcutaneous 
portion,  pass  through  the  fasciculi  of  the  orbicularis  oris,  reach  the  sub- 
mucous tissue,  where  they  cross  and  recross  one  another,  and  finally  pass 
into  the  membrana  propria,  where  they  end  in  fan-like  terminals.  The 
fasciculi  of  the  orbicularis  differ  somewhat  in  the  upper  and  lower  lips; 
in  the  former  the  bundles  are  strongly  developed  toward  the  angle  of 
the  mouth,  while  in  the  latter  the  median  bundles  are  the  strongest. 
The  labial  muscular  tissues  are  of  the  transversely  striated  variety.  The 
fibers  are  cylindric  in  form,  having  rounded  or  pointed  extremities  in 
the  interior,  and  broad  or  flattened  ends  where  they  come  in  contact 
with  the  periosteum.     When  examined  with  a  high  power  each  fiber 

34 


MUSCULAR    TISSUES    OF    THE    TUNGUE  35 

shows  alternately  broad  and  narrow  striae,  the  former  being  dim,  while 
the  latter  is  bright  in  appearance.  With  a  stronger  power  both  the  broad 
and  narrow  striae  are  seen  to  be  transversely  striated. 

Muscular  Tissues  of  the  Cheeks. — The  muscles  entering  into  the 
construction  of  the  lateral  walls  of  the  mouth  have  already  been  described 
in  part  I,  page  12,  giving  the  relations  existing  between  the  individual 
muscles,  together  with  the  general  disposition  of  the  various  fasciculi. 
Histologically  considered,  these  muscles  partake  of  all  the  characteristics 
of  striated  or  voluntary  muscular  tissue.  In  the  body  of  the  buccinator 
and  masseter  muscles  the  fibers  are  cylindric  and  have  definitely  pointed 
or  rounded  ends.  Near  their  termini,  particularly  in  the  latter  muscle, 
the  inner  extremities  of  the  terminal  fibers  are  pointed,  while  the  outer 
ends,  or  those  by  which  the  attachment  is  formed,  are  broad  and  rather 
flat. 

Muscular  Tissues  of  the  Soft  Palate. — The  disposition  of  the 
striated  muscular  tissue  of  the  soft  palate  is  extremely  complicated.  The 
azygos  uindce,  the  only  true  longitudinal  muscle  in  the  soft  palate,  has 
its  origin  from  aponeurosis  of  the  soft  palate  and  from  the  nasal  spine  of 
the  palate-bone,  the  fibers  passing  backward  upon  either  side  of  the 
median  line.  This  is  a  double  muscle,  and  near  its  point  of  origin  the 
two  portions  are  distinct  and  separated  by  a  definite  space,  but  upon 
reaching  the  base  of  the  uvula  they  become  closely  associated.  The 
fasciculi  do  not  continue  to  the  apex  of  the  uvula,  but  immediately  beyond 
the  center  of  its  length  are  thrown  out  fan-like  toward  the  sides,  terminat- 
ing in  a  manner  similar  to  the  fibers  of  the  lips.  In  passing  from  before 
backward  a  number  of  small  fasciculi  are  given  off,  which  reach  out 
laterally  and  traverse  the  glandular  lobes,  completely  surrounding  them, 
after  which  they  again  return  to  the  principal  fibers  at  the  median  line. 
The  palatopharyngeus  muscle  is  divisible  into  two  parts,  the  upper 
extremities  of  which  lie  partly  in  front  and  partly  behind  the  levator 
muscles.  The  greater  number  of  the  fibers  of  one  set,  situated  in  front 
of  the  levators,  form  a  curved,  flattened  aponeurosis.  The  fibrous 
border  of  the  hard  palate  serves  as  an  attachment  for  the  convex  border 
of  this  portion,  while  the  other  border,  which  is  concave,  is  directly 
toward  the  arch  of  the  levators.  The  fibers  of  the  palatopharyngeus, 
situated  behind  the  levators,  form  a  number  of  loose  fasciculi  inter- 
spersed by  fat-cells.  In  passing  toward  the  free  border  of  the  soft  palate 
the  fibers  become  much  more  delicate,  and,  separating,  some  course  in 
front  and  others  behind  this  muscle.  In  this  location  the  fibers  become 
closely  associated  with  the  glands,  and  either  end  here  or  are  continued 


36  ANATOMY 

to  the  submucosa,  or  even  to  the  membrana  propria  of  the  mucous 
membrane.  The  fibers  of  the  palatopharyngeus  unite  with  the  fibers  of 
the  levators,  and  an  arch-like  fasciculus  is  formed  by  this  union  which, 
subdividing,  passes  in  front  of  the  azygos  uvulae  to  the  opposite  side. 
All  of  these  fibers  run  outward  and  downward,  and  unite  with  the 
extremities  of  the  other  palatal  muscles,  the  fibers  of  which  are  some- 
what more  regularly  distributed.  Like  the  muscles  of  the  lips  and 
cheeks,  the  several  fasciculi  of  the  palatal  muscles  form  a  delicate 
plexus,  and  a  quantity  of  fatty  tissue  is  found  between  the  various 
fasciculi. 

Muscular  Tissues  of  the  Tongue. — The  tongue  is  divided  into 
two  equal  lateral  portions  by  a  median  septum — the  septum  Ungues. 
This  central  septum,  composed  of  a  vertical  layer  of  compact,  fibrous, 
connective  tissue,  extends  the  entire  length  and  depth  of  the  lingual 
median  line.  Beginning  at  the  hyoid  bone,  it  gradually  increases  in 
prominence  until  the  middle  of  the  organ  is  reached,  beyond  which  point 
it  becomes  less  pronounced  and  finally  disappears  near  the  tip.  The 
bundles  of  the  muscular  tissues  are  arranged  longitudinally,  transversely, 
and  vertically.  The  former  lie  immediately  beneath  the  mucous  mem- 
brane, including  the  superior  lingualis  above  and  the  inferior  lingualis 
below,  together  with  the  greater  part  of  the  styloglossus.  The  superior 
lingualis  extends  from  the  base  to  the  tip  of  the  organ,  and  by  short 
fasciculi  its  fibers  are  attached  to  the  overlying  tissues.  The  fibers  of 
this  muscle  are  placed  between  the  hyo-  and  styloglossi  muscles  of  the 
opposite  side,  both  of  which  overlap  the  fibers  of  the  lingualis  near  the 
base  of  the  tongue.  The  inferior  lingualis  also  gives  off  several  small 
fasciculi  and  fibers  to  the  mucous  membrane  beneath,  and  is  composed 
of  two  bands  which  reach  from  the  base  to  the  apex,  each  being 
placed  between  the  hyoglossus  and  genio-hyoglossus  muscles.  Th 
transverse  fibers,  which  are  placed  between  the  superior  and  inferior 
lingualis  muscles,  originate  from  the  septum  linguae,  and  form  the  bulk 
of  the  organ.  From  their  point  of  origin  these  fibers  course  outward  and 
upward  to  the  sides  of  the  tongue.  Those  fibers  which  are  vertically 
disposed  decussate  with  the  transverse  fibers,  and  pass  from  the  dorsum 
toward  the  under  surface  of  the  tongue,  the  fibers  curving  gracefully  with 
their  concavity  directed  toward  the  under  surface.  In  most  instances 
the  ascending  vertical  fibers,  as  well  as  the  transverse  fasciculi,  pass 
between  those  longitudinally  disposed  and  connect  with  the  submucosa. 


CHAPTER  III. 

The  Bones  of  the  Mouth:  The  Superior  Maxillae,  The  Palate  Bones, 
The  Inferior  Maxilla  or  Mandible. 

SUPERIOR  MAXILLARY  BONES. 

The  superior  maxillary  bones,  two  in  number,  one  on  each  side  of 
the  median  line  or  center  of  the  face,  are  irregular  in  shape,  and  may  be 
classed  as  the  largest  bones  of  the  face,  with  the  probable  exception  of 
the  mandible  or  inferior  maxilla.  From  the  central  position  which  they 
occupy  they  contribute  largely  to  the  bony  framework  of  this  portion  of 
the  skull.  They  are  not  only  instrumental  in  forming  the  major  portion 
of  the  roof  of  the  mouth  or  hard  palate,  but  assist  in  the  formation  of 
the  floor  of  the  orbit,  and  the  sides  and  base  of  the  nasal  chamber.  They 
furnish  a  solid  and  firm  foundation  for  the  sixteen  upper  teeth,  and  by 
their  variety  in  form  contribute  much  to  the  character  and  quality  of  the 
voice.  The  outer  or  facial  surface  of  these  bones  provides  attachment 
for  numerous  muscles.  Each  superior  maxillary  bone  presents  for 
examination  a  body,  four  surfaces,  and  four  processes.  The  body  may  be 
described  as  forming  an  irregular  triangle,  its  general  contour  depending 
much  upon  the  temperament  of  the  subject.  Within  the  body  of  the  bone 
is  an  irregular  cavity,  the  maxillary  sinus  or  antrum  of  Highmore. 

The  four  surfaces  of  the  body  of  the  bone  are  the  superior  or  orbital, 
the  lateral  or  facial,  the  proximal  or  nasal,  and  the  posterior  or  zygomatic. 

The  Superior  or  Orbital  Surface,  which  assists  in  forming  the 
greater  portion  of  the  floor  of  the  orbit,  is  slightly  concave  over  its  anterior 
two-thirds,  and  somewhat  convex  over  the  remaining  or  posterior  third. 
The  three  borders  of  this  surface  form  almost  an  equilateral  triangle,  and 
are  named,  as  indicated  by  their  location,  the  anterior,  the  posterior,  and 
the  mesial  or  proximal.  The  anterior  border  is  convex  from  before  back- 
ward and  slightly  concave  throughout  its  length.  That  portion  which 
forms  a  part  of  the  lower  border  of  the  completed  orbit  is  smooth,  while 
the  remaining  portion  is  roughened  to  form  an  articulation  with  the  malar 
bone.  The  posterior  border  extends  from  the  center  of  the  malar  process 
backward  and  inward  to  the  orbital  process  of  the  palate  bone,  which 

37 


38  ANATOMY 

articulates  with  the  superior  maxillary  at  this  point.  A  portion  of  this 
border,  together  with  the  orbital  process  of  the  palate  bone,  is  instru- 
mental in  forming  the  anterior  boundary  of  the  sphenomaxillary  fissure. 

The  mesial  or  proximal  border  is  marked  by  an  irregular  thin  edge, 
which  articulates  with  a  portion  of  two  bones,  the  lacrimal  anteriorly, 
and  the  os  planum  of  the  ethmoid  bone  posteriorly.  Only  the  posterior 
two-thirds  of  this  border  presents  an  articulating  edge,  the  remaining  or 
anterior  third  being  smooth  and  forming  the  commencement  of  the  lacri- 
mal groove,  which  in  the  articulated  skull  becomes  a  canal,  passing 
downward  and  backward  to  communicate  with  the  inferior  meatus  of 
the  nose.  Beginning  at  the  posterior  border  of  this  surface  and  running 
forward  will  be  found  a  deep  groove — the  infra-orbital  groove.  When 
near  the  center  of  the  surface,  this  groove  dips  down  and  is  covered  by  a 
layer  of  bone,  from  which  point  it  passes  forward  as  a  canal — the  infra- 
orbital canal — making  its  exit  at  a  point  about  £  of  an  inch  below  the 
border  of  the  orbit,  near  the  center  of  the  facial  surface  of  the  bone, 
the  foramen  thus  formed  being  the  infra-orbital  foramen.  Near  the 
root  of  the  nasal  process,  and  immediately  within  the  anterior  border 
of  this  surface,  is  a  small  depression  which  marks  the  origin  of  the  inferior 
oblique  muscle  of  the  eyeball. 

The  Lateral  or  Facial  Surface  (Fig.  15). — This  surface  is  made 
up  of  the  anterior  part  of  the  bone;  it  is  irregularly  concave,  and  pre- 
sents a  greater  variety  in  form  than  any  other  part  of  the  bone,  with  the 
single  exception  of  the  palatal  process.  It  is  bounded  above  by  the  infra- 
orbital ridge,  and  the  roughened  surface  of  the  malar  process  which  artic- 
ulates with  the  malar  bone;  below,  by  the  border  of  the  alveolar  process; 
anteriorly,  by  the  frail  concave  border  of  the  opening  into  the  nasal  cavity, 
the  anterior  nasal  spine,  and  the  perpendicular  margins  of  the  bone  be- 
neath. Posteriorly,  this  surface  is  separated  from  the  posterior  or  zygo- 
matic surface  by  a  strong  projecting  eminence,  the  malar  process. 

The  canine  fossa  is  a  deep  depression,  situated  almost  in  the  center 
of  this  surface,  the  bone  at  this  point  being  extremely  thin  and  closely 
related  to  the  floor  of  the  antrum.  The  concave  floor  of  this  fossa  is 
frequently  traversed  by  one  or  two  smaller  convex  ridges,  corresponding 
to  the  roots  of  the  bicuspid  teeth. 

The  canine  eminence  is  a  prominent  ridge  running  vertical  to  the 
body  of  the  bone  immediately  anterior  to  the  canine  fossa,  and  corre- 
sponding in  position  to  the  root  of  the  cuspid  tooth,  the  size  and  type  of 
the  tooth  having  much  to  do  with  its  extent  and  prominence.  This 
ridge  gives  origin  to  one  of  the  depressor  muscles  of  the  upper  lip,  and  also 


SUPERIOR   MAXILLARY    BONES 


39 


to  one  of  the  depressor  muscles  of  the  wing  of  the  nose.  The  incisive  or 
myrtiform  fossa  is  a  depression  found  between  the  canine  eminence  and 
the  inner  margin  of  the  bone.  The  depth  of  this  fossa  is  in  a  measure 
controlled  by  the  position  and  size  of  the  teeth,  and  by  the  amount  of 
prominence  in  the  canine  eminence. 

The  infra-orbital  foramen,  which  transmits  the  infra-orbital  nerves 
and  blood-vessels.,  is  immediatelv  below  the  center  of  the  infra-orbital 


Nasal  Process 


Infra-Orbital 
Foramen 


Canine  Fossa 

Nasal  Spine 
Facial  Surface 

Incisive  Fossa 

Canine  Eminence 


Orbital  Surface 


Malar  Process 


Posterior  Dental 
Canals 


Tuberosity 


Fig.   15. — Left  Superior  Maxilla,  Outer  or  Facial  Surface. 


ridge,  and  near  the  upper  margin  of  the  canine  fossa.  It  is  oval  in  form, 
and  faces  almost  directly  toward  the  median  line.  Between  this  foramen 
and  the  infra-orbital  ridge  is  the  point  of  origin  for  the  principal  levator 
muscle  of  the  upper  lip,  the  levator  labii  superioris  proprius.  The  whole 
extent  of  the  facial  surface  may  present  a  number  of  vertical  ridges,  or  the 
same  space  may  be  regular  and  smooth,  the  condition  being  controlled 
by  the  size  and  shape  of  the  tooth-roots  and  the  thickness  of  the  bone 
covering  them.  One  of  the  levator  muscles  of  the  angle  of  the  mouth, 
the  levator  anguli  oris,  is  attached  to  this  surface  near  the  upper  border 
of  the  canine  fossa. 


4o 


ANATOMY 


The  Proximal  or  Nasal  Surface  (Fig.  16). — Above,  this  surface 
presents  a  large,  irregular  opening  into  the  maxillary  sinus,  this  opening 
being  nearly  closed  in  the  articulated  skull  by  neighboring  bones.  In 
front  of  the  opening  into  the  sinus,  and  standing  perpendicular  from  the 
body  of  the  bone,  is  the  strong  ascending  plate  of  the  nasal  process, 
marked  near  its  lower  extremity  by  a  rough,  horizontal  ridge,  the  inferior 


Nasal  Process 


Ridge  for  Middle 
Turbinal 


Middle  Meatus 
Thin  plate  of 
bone  over  Lac- 
rymal  Groove 


Ridge  for  Infe- 
rior Turbinal 


Nasal  Spine 


Anterior  Pala- 
tine Groove 


Inferior  Meatus 


Posterior  Palatine 
Groove 


Palate  Proces 


Fig.  16. — Left  Superior  Maxilla,  Internal,  Proximal,  or  Nasal  Surface. 


turbinated  crest,  which  gives  attachment  to  the  inferior  turbinated  bone. 
The  smooth,  concave  surface  immediately  above  this  ridge  corresponds 
to  the  middle  meatus  of  the  nose,  and  forms  the  external  wall  of  that  pas- 
sage. Below  the  opening  into  the  sinus  and  the  nasal  process,  and  occupy- 
ing the  anterior  two-thirds  of  the  middle  of  this  surface,  is  a  large  semi- 
circular space,  forming  the  outer  wall  of  the  inferior  meatus  of  the  nose. 
Below  this  space,  and  projecting  inward  from  the  body  of  the  bone,  is 
the  palatal  process,   which   articulates   with   the   corresponding  process 


SUPERIOR  MAXILLARY  BONES  4 1 

of  the  opposite  bone.  At  the  anterior  superior  angle  of  the  nasal  surface, 
and  passing  downward  just  behind  the  nasal  process,  is  the  lacrimal 
groove.  In  the  articulated  skull  this  groove  becomes  a  canal,  the  lacri- 
mal canal,  the  ethmoid  and  the  inferior  turbinated  bones  assisting  in  its 
formation.  The  canal  passes  downward  and  slightly  backward,  and 
opens  into  the  inferior  meatus  of  the  nose.  It  is  about  1/2  of  an  inch  in 
length,  and  gives  passage  to  the  lacrimonasal  duct. 

The  lacrimal  tubercle  is  a  small  prominence  of  bone  formed  at  the 
junction  of  the  anterior  border  of  this  surface,  with  the  external  surface 
of  the  nasal  spine.  The  extended  portion  of  the  lacrimal  duct,  the 
lacrimal  sac,  finds  lodgment  at  this  point. 

The  posterior  palatine  or  palatomaxillary  canal  commences  near  the 
middle  of  the  posterior  border  of  this  surface,  appearing  in  the  disarticu- 
lated bone  as  a  groove,  and,  passing  downward  and  forward,  gives  pas- 
sage to  the  posterior  palatine  vessels  and  anterior  palatine  nerves.  The 
canal  is  made  complete  by  the  articulation  of  the  superior  maxillary  with 
the  vertical  plate  of  the  palate  bone.  On  the  posterior  portion  of  the  nasal 
surface,  extending  from  the  irregular  opening  into  the  antrum  downward 
to  a  point  opposite  the  palatal  process,  is  a  roughened  surface  about  1/2 
of  an  inch  in  width,  which  marks  the  extent  of  articulation  with  the 
palate  bone. 

The  proximal  or  nasal  surface  presents  four  borders — superior,  in- 
ferior, anterior,  and  posterior.  The  superior  border  is  irregular,  and 
articulates  with  the  lacrimal  and  ethmoid  bones.  The  inferior  border 
projects  inward,  and  forms  a  strong  horizontal  plate — the  palatal  process. 
This  process  defines  the  border  from  before  backward  to  the  posterior 
third,  at  which  point  it  is  marked  by  the  lower  border  of  the  roughened 
surface  which  articulates  with  the  palate  bone.  The  anterior  border  is 
sharp,  frail,  and  irregular  in  outline,  and  forms  the  free  margin  of  the 
opening  into  the  nasal  cavity.  The  posterior  border  is  marked  by  the 
inner  margin  of  the  zygomatic  surface,  being  smooth  upon  its  upper  half, 
and  roughened  upon  its  lower  half,  at  which  point  it  articulates  with  the 
palate  bone. 

The  Posterior  or  Zygomatic  Surface  (Fig.  16). — This  surface  is 
partly  convex  and  partly  concave,  and  is  bounded  above  by  a  well- 
defined  margin,  which  serves  as  the  dividing-line  between  this  and  the' 
superior  or  orbital  surface.  This  border  is  also  marked  by  a  roughened 
margin  on  the  posterior  portion  of  the  malar  process,  the  orbital  portion 
of  the  palate  bone  articulating  at  this  point.  The  major  portion  of  this 
border  is  smooth  and  rounded,  forming  the  lower  border  of  the  spheno- 


42  ANATOMY 

maxillary  fissure,  and  marked  by  a  notch,  the  commencement  of  the  infra- 
orbital groove.  The  outer  border  of  the  surface  is  formed  by  the  malar 
process,  and  by  a  line  drawn  from  this  point  directly  downward  to  the 
alveolar  process.  The  inner  border  is  smooth  and  somewhat  irregular 
above,  while  below  it  is  roughened  for  articulation  with  the  palate  bone. 

The  tuberosity,  which  also  forms  a  portion  of  the  inferior  border  of 
this  surface,  is  a  roughened  and  rounded  eminence  of  bone,  and  is  pene- 
trated by  a  number  of  nutrient  vessels,  which  enter  the  many  small  fora- 
mina at  this  point.  Between  the  tuberosity  and  the  body  of  the  zygomatic 
surface  are  several  large  apertures  leading  into  canals,  which  pass  into, 
and  give  nourishment  to,  the  substance  of  the  bone.  These  canals  trans- 
mit the  posterior  dental  blood-vessels  and  nerves,  one  of  which,  after 
passing  over  the  outer  wall  of  the  maxillary  sinus,  unites  with  the  anterior 
dental  canal.  The  tuberosity  is  posterior  to,  and  above,  the  third  molar 
tooth,  in  some  instances  extending  directly  backward  from  this  tooth  for 
the  distance  of  half  an  inch  or  more,  but  usually  the  tooth  penetrates  the 
base  of  the  tuberosity,  leaving  but  a  thin  layer  of  bone  posterior  to  it. 

The  inferior  border  of  the  posterior  or  zygomatic  surface  is  formed 
by  that  portion  of  the  alveolar  process  which  supports  the  second  and 
third  molar  teeth. 

The  bone  presents  four  processes  for  examination — the  nasal,  the 
malar,  the  palatal,  and  the  alveolar. 

The  nasal  process  is  a  strong,  irregular  piece  of  bone,  standing  ver- 
tically above  the  body  of  the  bone  proper,  and  forming  the  lateral  boundary 
of  the  nose.  This  process  is  greatly  increased  in  strength  by  the  infra- 
orbital ridge  joining  it  at  or  near  its  base,  and  ascending  its  external 
anterior  surface  to  some  extent.  That  portion  of  the  process  posterior 
to  its  junction  with  the  infra-orbital  ridge  assists  in  forming  the  inner 
wall  of  the  orbit. 

The  external  or  anterior  surface  of  the  nasal  process  is  marked  by  a 
number  of  shallow  grooves,  traces  of  the  development  of  the  bone. 
Scattered  over  this  surface  are  a  number  of  small  foramina,  the  entrances 
to  minute  canals  transmitting  nutrient  vessels  to  the  body  of  the  bone. 
This  surface  gives  origin  to  one  of  the  lip  muscles,  the  levator  labii  superi- 
oris  alaeque  nasi. 

The  internal  surface  of  the  nasal  process  is  usually  described  as  in- 
cluding all  that  portion  between  the  superior  border  and  the  floor  of  the 
anterior  nares.  The  surface  is  marked  by  two  concave  portions  and 
two  ridges.  The  two  ridges  divide  the  surface  into  three  parts — the 
superior  meatus,  the  middle  meatus,  and  the  inferior  meatus  of  the  nose. 


SUPERIOR    MAXILLARY    BONES  43 

The  superior  meatus  is  the  smallest  of  the  three,  and  occupies  the  slightly 
concave  space  above  the  superior  ridge.  The  middle  meatus,  partly 
concave,  and  partly  convex,  includes  the  space  between  the  superior 
and  the  inferior  ridges,  and  extends  from  the  free  margin  of  the  bone  in 
front  to  the  lacrimal  groove  behind.  The  inferior  meatus,  which  is 
much  the  largest,  occupies  all  that  concave  surface  between  the  inferior 
ridge  above  and  the  palatal  process  below,  and  extends  from  the  anterior 
margin  of  the  bone  backward  to  the  point  of  articulation  with  the  palate 
bone.  The  two  ridges  previously  referred  to  are  known  as  the  superior 
turbinated  crest,  which  articulates  with  the  middle  turbinated  bone,  and 
the  inferior  turbinated  crest,  which  articulates  with  the  inferior  turbinated 
bone. 

The  malar  process  is  a  large,  irregular  portion  of  bone  situated 
at  the  angle  of  separation  between  the  facial  and  zygomatic  surfaces, 
and  presents  a  triangular,  roughened  surface  for  articulation  with  the 
malar  bone.  The  superior  boundary  of  this  process  is  formed  by  the 
orbital  surface  and  the  outer  end  of  the  infra-orbital  ridge;  the  inferior 
boundary  may  be  marked  by  an  irregular  imaginary  line  running  from 
the  upper  margin  of  the  canine  fossa  to  a  point  between  the  first  and  second 
molar  teeth,  while  the  posterior  inferior  boundary  may  be  traced  from 
the  outer  superior  angle  of  the  zygomatic  surface  downward  and  forward 
to  the  point  above  referred  to.  This  process,  as  well  as  the  nasal  process, 
is  subject  to  much  variety  in  form  and  general  outline.  The  malar 
process,  assisting  as  it  does  in  forming  what  is  commonly  called  the  cheek 
bone,  is  particularly  variable  in  size,  and  in  certain  types  and  races  it  is 
so  prominent  as  to  become  a  controlling  feature  in  the  facial  form.  One 
of  the  muscles  of  mastication — the  masseter — has  a  portion  of  its  origin 
from  the  malar  process. 

The  palatal  process  is  more  directly  interested  in  the  formation  of 
the  cavity  of  the  mouth  than  any  other  portion  of  the  superior  maxillary 
bone.  By  articulating  with  its  fellow  of  the  opposite  side,  it  forms  about 
three-fourths  of  the  hard  palate  or  roof  of  the  mouth,  the  remaining  fourth 
being  formed  by  a  portion  of  the  palate  bones.  It  is  thick  and  strong,  and 
projects  horizontally  inward  from  the  inner  surface  of  the  body  of  the 
bone.  It  presents  two  surfaces  for  examination — a  superior  or  nasal 
surface,  and  an  inferior  or  oral  surface.  The  superior  or  nasal  surface 
is  smooth  and  more  or  less  concave,  and  forms  the  floor  of  the  nares. 
The  inferior  or  oral  surface  is  also  concave,  but  is  much  roughened  by 
numerous  small  projections,  between  which  are  lodged  the  mucous 
glands.     Upon  the  anterior  portion  of  this  surface   are   a  number  of 


44  ANATOMY 

small  foramina,  which  mark  the  entrance  to  numerous  small  canals 
giving  passage  to  nutrient  vessels  to  supply  the  body  of  the  bone.  Near 
the  center  of  the  posterior  third  are  the  anteroposterior  grooves,  which 
accommodate  the  posterior  palatine  nerves  and  blood-vessels.  This 
process  also  presents  for  examination  three  borders  and  various  other 
points  of  interest.  The  three  borders  are  the  interior,  posterior,  and 
mesial.  The  anterior  border  is  thick  and  somewhat  irregular;  the  posterior 
border  is  thin  and  frail,  and  articulates  with  a  portion  of  the  palate 
bone.  The  mesial  border  presents  a  wide  articulating  surface  in  front, 
behind  it  is  narrow,  the  whole  extent  of  this  border  articulating  with  the 
corresponding  process  of  the  opposite  bone. 

The  Nasal  Spine. — At  the  anterior  superior  angle  of  the  palatal 
process  is  a  well-defined  spine — the  nasal  spine — being  formed  by  a  pro- 
longation of  the  process  beyond  the  level  of  the  facial  surface  of  the  bone. 
This  process,  when  articulated  with  its  fellow  of  the  opposite  side,  forms 
the  base  of  the  nose. 

The  Nasal  Crest. — Beginning  at  the  base  of  the  nasal  spine,  and  ex- 
tending backward  along  the  median  border  of  the  bone,  is  a  sharp, 
irregular  ledge  of  bone,  the  nasal  crest.  This  portion  of  the  process 
articulates  with  the  vomer. 

The  incisor  crest  is  a  continuation  of  the  nasal  crest  anteriorly,  pro- 
jecting beyond  the  nasal  spine  in  the  form  of  a  sharp,  spear-like  point. 

The  incisive  foramen,  or  foramen  of  Stenson,  is  situated  immediately 
back  of  the  incisor  crest,  and  leads  downward  and  forward  from  the  nasal 
chamber  toward  the  mouth,  entering  that  cavity  just  back  of  the  central 
incisor  tooth.  This  passage  in  the  single  bone  is  a  simple  groove,  but 
in  the  articulated  skull  it  becomes  the  anterior  palatine  canal,  which, 
after  passing  downward,  opens  on  the  nasal  surface  of  the  palatal  process 
by  four  foramina — the  incisive  foramina,  and  the  foramina  of  Scarpa,  or 
the  naso-palatine  foramina.  These  foramina  transmit  the  naso-palatine 
nerves. 

The  palatal  process  of  the  superior  maxilla  is  subject  to  a  greater 
variety  in  form  than  any  other  portion  of  the  bone,  this  variation  in  the 
articulated  skull  being  the  cause  of  the  many  different  curves  assumed 
by  the  roof  or  dome  of  the  mouth. 

The  Alveolar  Process. — This  process  forms  the  lower  margin  of 
the  bone,  and  extends  from  the  base  of  the  tuberosity  behind  to  the 
median  line  in  front,  at  which  point  it  articulates  with  the  same  process 
of  the  opposite  bone.  It  has  an  outer  and  an  inner  margin  corresponding 
to  the  buccal  and  palatal  surfaces  of  the  roots  of  the  teeth,  which  are 


SUPERIOR    MAXILLARY    BONES  45 

firmly  imbedded  in  it.  Its  general  form  from  before  backward  is  that  of 
a  gradual  curve,  somewhat  variable  in  different  bones,  the  extent  of  this 
variation  depending  on  the  type  or  race  to  which  the  bone  belongs.  The 
body  of  the  process  is  made  up  of  an  outer  and  an  inner  plate,  which  are 
connected  by  numerous  septa  of  cancellated  bone.  The  outer  plate  of 
the  process  is  continuous  with  the  facial  and  zygomatic  surface  of  the  body 
of  the  bone,  and  assists  in  forming  these  surfaces.  It  is  quite  thin  and 
frail,  and  the  position  of  the  alveoli  beneath  are  well  shown  by  the  numer- 
ous vertical  ridges  upon  it.  The  inferior  margin  of  the  outer  plate  is 
reinforced  by  an  additional  thickness  of  bone,  forming  the  border  of  the 
alveolar  sockets.  The  inner  plate  of  the  alveolar  process  is  much 
heavier  and  stronger  than  the  outer  plate,  and  extends  from  the  margins 
of  the  alveoli  below  to  the  palatal  process  above.  The  inferior  margin 
of  this  plate  is  not  reinforced  except  in  the  region  of  the  molars.  The 
construction  of  the  inner  plate  is,  to  a  great  degree,  controlled  by  the 
shape  and  position  of  the  palatal  process.  In  the  lymphatic  tempera- 
ment this  process,  when  articulated  with  its  fellow,  forms  a  flat  or 
shallow  dome  to  the  oral  cavity,  and  in  so  doing  gradually  curves  into 
the  alveolar  process,  giving  it  additional  thickness.  The  depth  of  the 
process  in  this  type  is  not  great,  and  the  roots  of  the  teeth  are  short 
and  heavy  in  proportion.  In  the  bilious  temperament  the  inner  al- 
veolar plate  is  deep  and  abrupt,  extending  from  the  inferior  margin 
upward  in  almost  a  perpendicular  direction  to  the  palatal  process  which 
joins  it  almost  at  right  angles.  The  alveolar  process  gives  origin  to  one 
of  the  cheek  muscles — the  buccinator — which  is  attached  to  the  outer 
plate  near  its  upper  margin,  and  directly  over  the  space  occupied  by  the 
second  bicuspid  and  first  molar  teeth. 

The  Alveoli  or  Tooth  Sockets. — These  cavities,  which  are  variable 
in  number,  are  formed  by  the  outer  and  inner  plate  of  the  alveolar  process, 
and  by  numerous  connecting  septa  of  bone  placed  between  the  two  plates. 
The  shape  and  depth  of  each  cavity  is  regulated  by  the  form  and  length 
of  the  roots  of  the  teeth  which  they  support.  The  first  socket,  or  that 
next  to  the  median  line,  gives  support  to  the  central  incisor  tooth.  It 
forms  almost  a  perfect  cone,  and  has  an  average  depth  of  half  an  inch. 
Its  lower  border  is  circular,  and  the  anterior  or  labial  portion  describes 
a  larger  circle  than  the  posterior  or  palatal  half.  The  mesial  and  distal 
walls  are  somewhat  flattened.  The  second  cavity,  proceeding  backward 
from  the  median  line,  supports  the  lateral  incisor  tooth.  It  is  also  conic, 
but  somewhat  smaller  than  the  preceding.  It  is  seldom  over  3/8  to  5/16 
of  an  inch  in  depth.     It  is  much  flattened  on  its  mesial  and  distal  walls, 


46  ANATOMY 

giving  the  appearance  of  an  oblong,  rather  than  a  round,  cavity  in  trans- 
verse section.  This  socket,  as  well  as  that  for  the  central  incisor,  occupies 
an  almost  vertical  position  in  the  process.  Very  frequently  the  socket 
for  the  lateral  incisor  presents  a  slight  distal  curve  at  its  upper  extremity. 
The  third  socket,  or  that  giving  support  to  the  cuspid  tooth,  is  much 
larger  and  deeper  than  either  of  those  previously  described..  It  extends 
upward,  inward,  and  backward  with  an  average  depth  of  5/8  to  3/4  of 
an  inch.  In  transverse  section,  its  labial  wall  presents  a  much  larger 
circle  than  its  palatal  margin.  The  labial  and  distal  walls  are  much 
flattened  and  somewhat  convex.  The  general  direction  of  this  socket  is 
slightly  to  the  distal.  The  socket  which  supports  the  first  bicuspid  is 
usually  divided  from  mesial  to  distal  by  a  thin  septum  of  bone,  thus  form- 
ing an  outer  or  buccal  socket,  and  an  inner  or  palatal  socket.  This 
division  seldom  exists  to  the  full  depth  of  the  cavity,  but  usually  begins 
about  midway  of  its  length.  The  lower  margin  of  this  socket  is  oblong  or 
egg-shaped,  its  outer  or  buccal  portion  forming  a  larger  curve  than  its 
palatal.  The  lateral  walls  are  slightly  concave  or  flattened,  until  the  point 
of  separation  is  reached,  when  they  become  more  circular,  the  alveoli 
above  this  point  becoming  cone-shaped.  It  is  not  uncommon  for  this 
socket  to  be  a  single  cavity,  and  when  thus  formed  it  resembles  a 
flattened  cone,  with  the  buccal  and  palatal  margins  rounded.  The 
next  socket  gives  support  to  the  second  bicuspid  tooth,  in  most  in- 
stances being  a  single  cavity,  but  in  rare  instances  it  is  divided  near 
its  upper  extremity.  In  general  outline  it  resembles  the  socket  for  the 
first  bicuspid. 

The  socket  for  the  first  molar  is  much  larger  than  any  of  those  pre- 
viously described;  its  inferior  margin  presents  a  circular  outline  on  its 
buccal  and  palatal  portions,  the  former  curve  being  larger  than  the  latter. 
The  mesial  and  distal  walls  are  flattened  and  slightly  concave.  The 
upper  three-fourths  of  this  socket  is  divided  into  three  separate  compart- 
ments, being  so  arranged  that  two  are  upon  the  buccal  and  one  upon  the 
palatal  side.  The  septa  separating  the  two  buccal  cavities  from  the  pala- 
tal cavity  are  heavy  and  strong,  while  that  placed  between  the  two  buccal 
sockets  is  thin  and  frail.  The  two  buccal  cavities  are  usually  flattened 
upon  their  mesial  and  distal  sides.  The  palatal  socket  is  larger  and 
somewhat  deeper  than  the  buccal,  the  average  depth  of  all  being  about 
1/2  of  an  inch.  The  socket  for  the  second  molar  is  similar  in  most  respects 
to  that  for  the  first  molar,  except  that  it  is  somewhat  smaller.  The  same 
description  might  answer  for  the  third  molar  socket,  which  in  general 
is  similar  to  the  alveoli  for  the  other  molars.     It  is  smaller  than  the  second 


SUPERIOR    MAXILLARY    BONES  47 

molar  socket,  and  may  be  a  single  cavity,  or  it  may  be  divided  into  three 
compartments. 

Articulations. — The  superior  maxillary  bone  articulates  with  its 
fellow  of  the  opposite  side,  with  the  frontal,  lacrimal,  ethmoid,  palate, 
vomer,  malar,  and  inferior  turbinated  bones.  Occasionally  it  articulates 
with  the  sphenoid  bone. 

Attachment  of  Muscles. — The  muscles  attached  to  this  bone  are 
eleven  in  number,  and  are  as  follows : 

Compressor  nares,  Internal  pterygoid, 

Orbicularis  oris,  Orbicularis  palpebrarum, 

Levator  labii  superioris  alaeque  Levator  labii  superioris  proprius, 

nasi,  Inferior  oblique, 

Levator  anguli  oris,  Buccinator, 

Depressor  alse  nasi,  Masseter. 

Blood-supply. — The  maxilla  receives  its  vascular  supply  from 
numerous  large  arteries.  They  are  derived  from  the  alveolar,  infra- 
orbital, nasopalatal,  descending  palatal,  ethmoidal,  nasal,  frontal,  and 
facial  branches. 

Development. — The  superior  maxilla  arises  from  four  points  of 
ossification,  which  are  deposited  in  membrane.  These  four  centers 
make  their  appearance  as  early  as  the  eighth  fetal  week,  this  eariy  begin- 
ning making  it  somewhat  difficult  to  accurately  follow  its  growth.  The 
four  centers  are  named,  as  located,  premaxillary,  maxillary,  malar,  and 
prepalatal.  The  premaxillary  nucleus  gives  rise  to  the  incisive  portion 
of  the  bone,  or  that  part  supporting  the  incisor  teeth.  During  early  life 
this  division  of  the  bone  is  separated  from  the  body  of  the  bone,  and  is 
known  as  the  premaxillary  portion  (Fig.  17).  Union  between  the  pre- 
maxillary portion  and  the  maxilla  proper  takes  place  about  birth,  and  the 
suture  thus  formed  is  visible  on  the  facial  surface  until  the  sixth  or  seventh 
year,  and  on  the  palatine  surface  until  the  adult  period.  The  palatal 
suture  extends  as  far  back  as  the  posterior  border  of  the  anterior  palatal 
canal.  This  nucleus  also  sends  a  narrow  process  upward  which  forms 
part  of  the  outer  boundary  of  the  anterior  narial  aperture.  On  the 
palatal  aspect  it  furnishes  a  speculum  which  surrounds  the  anterior  and 
mesial  walls  of  Stenson's  canal.  The  posterior  limit  of  the  premaxillary 
portion  is  indicated  by  the  suture  on  the  palatal  surface.  The  maxillary 
nucleus  forms  the  greater  portion  of  the  body  of  the  true  maxilla  and  the 
nasal  process.  The  malar  center  gives  origin  to  the  malar  process, 
and  all  that  portion  external  to  the  infra-orbital  groove.     The  prepalatine 


48 


ANATOMY 


center  gives  rise  to  the  nasal  surface  of  the  bone  and  that  portion  of  the 
palatal  process  posterior  to  Stenson's  canal. 

Development  of  the  Alveolar  Process. — This  process  is  represented 
at  birth  by  the  walls  of  a  deep  groove,  in  which  are  lodged  the  partly 
calcified,  deciduous  teeth  and  the  germs  of  most  of  the  permanent  teeth 
(see  Development  of  the  Teeth). 

The  growth  of  the  process  continues  with  the  growth  of  the  teeth 
until,  finally,  at  about  the  seventh  month  after  birth,  the  dental  organs 


Palatal  Portion 


Fig. 


Malar  Portion 


-Left  Superior  Maxillary,  about  the  Third  Year,  Enlarged. 


are  completely  encased  within  its  walls.  With  the  decalcification  of  the 
roots  of  the  deciduous  teeth  comes  the  loss  of  the  process  surrounding 
them,  and,  as  the  permanent  teeth  advance  to  take  their  place  in  the 
arch,  the  process  is  again  built  up  about  their  roots. 

The  Maxillary  Sinus,  or  Antrum  of  Highmore*  (Fig.  16).— This 
is  a  large  cavity  situated  within  the  body  of  the  maxilla.  Its  general 
shape  is  that  of  a  pyramid,  with  its  base  directed  toward  the  median  line, 
or  nasal  surface,  its  apex  pointing  toward  and  extending  into  the  malar 
process,  and,  in  some  instances,  penetrating  the  malar  bone.  The  size 
of  the  cavity  varies  in  different  subjects  and  in  the  opposite  bone  of 

*  Described  separately,  in  preference  to  including  in  general  description  of  the  bone. 


SUPERIOR  MAXILLARY  BONES 


49 


the  same  subject.  The  average  capacity  is  about  three  fluidrams,  but 
this  may  be  increased  to  six  or  eight  fluidrams.  The  size  of  the  bone  and 
the  prominence  of  the  malar  process  control,  in  a  measure,  the  size  of 
the  cavity;  but  not  infrequently  the  largest  bone  will  present  the  smallest 
sinus.  Sex  also  appears  to  exert  a  controlling  influence  over  the  capacity 
of  the  cavity,  it  being  greater  in  the  male  than  in  the  female.  In  youth 
the  cavity  is  quite  small,  the  walls  being  much  thicker  proportionately 
than  in  the  adult.  The  walls  of  the  sinus  in  the  matured  subject  are 
quite  thin  and  frail,  and  are  four  in  number.     The  superior  wall  is  formed 


Fig.  18. — Developing  Maxillary  Bones  about  the  Fifth  Month  after  Birth. 

by  a  thin  plate  of  bone,  the  floor  of  the  orbit.  This  surface  is  almost  flat, 
and  serves  as  a  roof  to  the  cavity.  Near  the  anterior  margin  of  this  sur- 
face is  a  thick  rib  of  bone  which  marks  the  course,  and  forms  one  of  the 
walls  of  the  infra-orbital  canal. 

The  inner  wall,  or  that  looking  toward  the  nasal  surface,  is  formed 
by  the  thin  bony  layer  separating  this  cavity  from  that  of  the  nares.  The 
outer  or  lateral  surface,  formed  by  the  facial  and  zygomatic  surfaces  of 
the  bone,  is  smooth,  and  convex  from  before  backward.  Near  the  center 
of  this  surface  the  cavity  may  penetrate  the  malar  process,  and  in  the  dis- 
articulated skull  would  present  an  opening  at  this  point.  The  inferior 
wall  is  formed  by  the  alveolar  process,  and  is  marked  by  a  number  of 
irregular  eminences  corresponding  to  the  roots  of  the  neighboring  teeth. 
The  teeth  referred  to  are  generally  the  first  and  second  molars,  and  occa- 

4 


50  ANATOMY 

sionally  the  second  bicuspid.  It  is  not  unusual  for  the  roots  of  one  or  more 
of  these  teeth  to  penetrate  the  floor  of  the  sinus,  in  consequence  of  which 
the  lining  membrane  of  the  cavity  may  suffer  disease  generated  in  the 
teeth. 

The  inferior  wall  is  much  the  strongest  of  the  four,  and,  besides  the 
unevenness  of  the  surface  produced  by  the  tooth-roots,  it  frequently 
supports  a  number  of  thin,  bony  partitions,  which  may  completely  or 
partly  divide  the  floor  of  the  cavity  into  numerous  small  compartments. 

The  posterior  portion  of  the  lateral  wall  is  marked  by  the  posterior 
dental  canals,  which  give  passage  to  the  posterior  nerves  and  blood-vessels. 
In  like  manner  the  anterior  portion  of  the  lateral  wall  is  grooved  for  the 
reception  of  the  anterior  dental  nerves  and  blood-vessels.  Upon  the  inner 
wall,  or  that  forming  the  base  of  the  pyramid,  is  an  opening  which  commu- 
nicates with  the  middle  meatus  of  the  nose.  In  the  articulated  skull  this 
opening  is  quite  small,  being  from  1/8  to  1/4  of  an  inch  in  diameter. 
The  correct  idea  of  this  opening  cannot  be  obtained  by  studying  the 
individual  bone,  as  the  numerous  perforations  then  to  be  observed  are 
closed  or  partly  closed  by  articulation  with  adjacent  bones.  The  mucous 
membrane  lining  the  nasal  cavity  enters  the  sinus  through  the  small 
aperture  above  referred  to,  and  forms  a  continuous  lining  over  its  entire 
surface.  Cryer  has  thrown  much  light  upon  the  relations  of  the  maxillary 
sinus  to  the  mouth  and  teeth,  and  he  has  demonstrated  beyond  a  doubt 
that  the  relationship  existing  between  the  parts  is  susceptible  to  extensive 
variation.  He  has  shown  that  in  some  instances  the  cavity  upon  one 
side  will  be  large,  with  its  floor  broken  by  the  tooth-roots,  while  that  upon 
the  opposite  side  will  be  extremely  small  and  far  removed  from  the  root 
apices.  In  fact,  these  researches  have  so  revolutionized  the  subject 
under  consideration  that  the  foregoing  description  is  only  reliable  in  so 
far  as  it  treats  of  the  conditions  most  frequently  met  with. 

THE  PALATE  BONE. 

The  palate  banes  (Fig.  19),  two  in  number,  are  situated  immediately 
posterior  to  the  two  maxillae,  and  with  them  complete  the  hard  palate 
They  also  assist  in  forming  the  boundaries  of  the  orbital  and  nasal  cavities, 
the  sphenomaxillary,  the  sphenopalatine,  and  the  pterygoid  fossa,  the 
sphenomaxillary  fissure,  the  posterior  ethmoidal  cells,  and  the  maxillary 
sinus.  When  in  position  in  the  skull,  these  bones  are  wedged  between 
the  maxillae  and  the  sphenoid  bone.  They  are  rectangular  in  outline,  and 
each  bone  presents  for  examination  a  horizontal  and  a  vertical  plate,  a 
tuberosity,  and  two  processes,  the  orbital  and  the  sphenoid. 


THE    PALATE    BOXE 


51 


The  horizontal  plate,  smaller  than  the  vertical,  assists  in  forming 
the  hard  palate,  and  corresponds  to  the  palatal  process  of  the  maxilla. 
In  entering  into  the  construction  of  the  hard  palate  the  form  of  this  plate 
varies  to  the  same  degree  as  the  palatal  plate  of  the  maxilla.  In  general, 
it  is  described  as  quadrilateral  in  shape,  having  two  surfaces  and  four 
borders.  The  superior  surface,  which  is  concave  from  side  to  side,  forms 
the  posterior  floor  of  the  nasal  chamber.  The  inferior  surface  completes 
the  hard  palate  posteriorly,  and  presents,  near  its  posterior  border,  a 
transverse  ridge  for  the  attachment  of  one  of  the  muscles  of  the  soft 
palate  (the  tensor  palati) ;  the  anterior  border  is  serrated  for  articulation 


eVTA<-  Pftoc, 


Fig.  19. 


'serosity 

I 

PTERYGOID    FOSSA 

-The  Two  Palate  Bones  in  their  Natural  Position, 
Dorsal  Mew.     (Testul.) 


with  the  palatal  process  of  the  maxilla.  The  posterior  border  is  free, 
curved,  and  sharp,  and  marks  the  posterior  boundary  of  the  hard  palate. 
At  the  median  line  this  border  terminates  in  a  sharp  point,  which,  when 
articulated  with  the  corresponding  bone  of  the  opposite  side,  forms  the 
posterior  nasal  spine;  to  this  point  the  azygos  uvulae  muscle  is  attached. 
The  external  border  is  situated  just  below  the  junction  of  the  horizontal 
and  vertical  plates.  In  this  portion  is  a  groove  which  assists  in  forming 
a  portion  of  the  posterior  palatal  canal.  The  internal  border  is  broad 
and  serrated  for  articulation  with  its  fellow  of  the  opposite  side.  When 
the  palate  bones  are  in  position  in  the  skull,  these  borders  form  a  ridge, 
continuing  the  crest  formed  by  the  palatal  process  of  the  maxilla,  this 
crest  receiving  the  inferior  border  of  the  vomer. 

The  vertical  plate  is  thin  and  frail  and  extends  from  the  floor  of 
the  nasal  chamber  below  to  the  upper  extremity  of  the  sphenopalatine 
notch  above.     It  has  two  surfaces  and  four  borders. 


52  ANATOMY 

The  external  surface  is  roughened  for  articulation  with  the  maxilla, 
excepting  a  small  triangular  surface  near  the  upper  extremity,  which 
forms  a  portion  of  the  sphenomaxillary  fossa,  and  a  small  portion  near 
the  middle  of  the  surface  close  to  the  anterior  border,  which  forms  a 
portion  of  the  wall  of  the  maxillary  sinus.  Near  the  posterior  boundary 
of  this  surface  is  a  vertical  groove,  which  forms,  when  articulated  with  the 
maxilla,  the  posterior  palatal  canal,  transmitting  the  descending  palatal 
nerves  and  vessels. 

The  internal  surface  is  divided  into  three  shallow  depressions  by  two 
transverse  ridges — the  superior  and,  inferior  turbinated  crests.  The 
lower  depression  thus  formed  assists  in  the  construction  of  a  portion  of 
the  interior  meatus  of  the  nose.  The  crest  immediately  above  this  depres- 
sion articulates  with  the  inferior  turbinated  bone.  The  central  depres- 
sion, the  largest  of  the  three,  forms  a  portion  of  the  middle  meatus  of  the 
nose,  the  crest  above  articulating  with  the  middle  turbinated  bone.  The 
superior  depression — much  smaller  but  deeper  than  either  of  those  pre- 
viously described — forms  a  large  part  of  the  superior  meatus.  The  an- 
terior border  of  the  vertical  plate  is  thin  and  sharp,  the  inferior  turbinated 
crest  protruding  near  the  center  of  the  border,  and  forming  the  maxillary 
process.  This  process  assists  in  closing  the  maxillary  sinus  by  being 
received  into  the  maxillary  fissure  of  the  maxilla.  At  the  upper  extrem- 
ity of  this  border  is  the  orbital  process,  which  presents  for  examination 
live  surfaces,  three  of  which  are  articular.  The  anterior  or  maxillary 
surface  is  directed  outward,  upward,  and  downward.  It  is  oblong  in 
form  and  articulates  with  the  posterior  superior  angle  of  the  inner  surface 
of  the  maxilla.  The  posterior  or  sphenoidal  surface  is  directed  back- 
ward, upward,  and  inward,  and  articulates  with  the  vertical  plate  of  the 
ethmoid  bone.  The  superior  or  orbital  surface  is  triangular  in  form, 
extending  upward  and  outward,  forming  the  posterior  angle  of  the  floor 
of  the  orbit.  The  external  or  zygomatic  surface  is  smooth,  oblong,  and 
directed  outward,  backward,  and  downward,  forming  a  portion  of  the 
sphenomaxillary  fossa. 

The  posterior  border  of  the  vertical  plate  is  irregular  and  serrated, 
and  comes  into  relation  with  the  internal  pterygoid  process,  terminating 
below  in  a  prominent  tuberosity.  This  presents  three  grooves  or  flutes. 
The  inner  receives  the  internal  pterygoid,  the  outer  the  external  pterygoid 
process,  while  the  middle  groove  completes  the  pterygoid  fossa,  and 
gives  attachment  to  a  portion  of  the  internal  pterygoid  muscle.  This 
process  also  gives  rise  to  the  superior  constrictor  of  the  pharynx. 
Passing  through  the  tuberosity  are  a  number  of  small  canals,  those  on  the 


THE    PALATE    BONE  53 

nasal  side  being  the  accessory  palatal  canals.  Near  the  junction  of  the 
tuberosity  with  the  horizontal  plate  is  the  opening  of  the  posterior  palatal 
canal,  and  beyond  this  the  small  external  palatal  canals. 

The  sphenoidal  process  is  at  the  superior  end  of  the  posterior  border. 
It  is  variable  in  shape  and  curves  upward,  backward,  and  inward.  It 
presents  a  superior,  an  external  and  an  internal  surface,  and  two  borders 
— an  anterior  and  a  posterior. 

The  superior  surface,  the  smallest  of  the  three,  is  marked  by  a  groove, 
which  assists  in  forming  the  sphenopalatine  canal.  This  surface  articu- 
lates with  the  horizontal  portion  of  the  sphenoidal  turbinated  bone. 
The  external  surface  assists  in  forming  the  sphenomaxillary  fossa  by  its 
anterior  portion,  while  the  posterior  portion  is  rough  for  articulation 
with  the  pterygoid  plate  of  the  ethmoid  bone.  The  internal  surface  is  in- 
strumental in  forming  a  portion  of  the  outer  wall  of  the  posterior  nafes, 
and  for  this  purpose  is  smooth  and  concave.  The  anterior  border 
forms  the  posterior  margin  of  the  sphenopalatine  notch.  The  posterior 
border  is  serrated,  and  articulates  with  the  inner  surface  of  the  pterygoid 
process. 

The  superior  border  of  the  vertical  plate  is  divided  by  a  deep  notch  or 
foramen,  which  divides  the  orbital  from  the  sphenoidal  process.  This 
opening  is  the  sphenopalatine  notch  or  foramen,  and  transmits  the  spheno- 
palatine vessels  and  nerves  from  the  sphenopalatine  fossa  to  the  nasal 
chamber. 

The  inferior  border  of  the  vertical  plate  joints  the  external  border  of 
the  horizontal  plate.  Extending  downward  and  backward  from  the 
inferior  and  posterior  borders  is  the  pyramidal  process,  the  borders 
of  which  are  serrated  for  articulation  with  both  pterygoid  plates  of  the 
sphenoid  bone. 

Articulations. — The  palate  bone  articulates  with  the  sphenoid, 
superior  maxilla,  sphenoidal  turbinated,  inferior  turbinated,  ethmoid, 
and  with  its  fellow  of  the  opposite  side. 

Attachment  of  Muscles. — The  following  muscles  are  attached  to 
the  palate  bone: 

Tensor  palati,  Internal  pterygoid, 

Azygos  uvulae,  Superior  constrictor  of  pharynx. 

Blood-supply. — The  arteries  which  supply  this  bone  are  derived 
from  branches  of  the  descending  palatine,  the  sphenopalatine,  and 
pterygopalatine. 

Development. — The  palate  bone  is  developed  from  a  single  center 


54 


ANATOMY 


deposited  in  membrane.  This  center  makes  its  appearance  about  the 
eighth  or  ninth  fetal  week,  near  the  line  of  junction  between  the  horizon- 
tal and  vertical  plates.  At  birth  these  plates  are  about  the  same  length, 
but  soon  after  this  period,  when  the  nasal  sinuses  increase  in  height, 
the  vertical  plate  begins  to  lengthen,  and  continues  to  do  so  until  it  be- 
comes nearly  double  the  length  of  the  horizontal  plate. 


INFERIOR  MAXILLARY  BONE. 

The  Inferior  Maxillary,  Mandible,  or  Lower  Jaw  Bone  (Fig.  20). — 
This  bone,  having  no  osseous  union  with  the  skull  proper,  may  be  con- 


Fig.  20. — The  Mandible  or  Inferior  Maxilla.  Right  Side,  External  or  Facial  Surface. 
a,  Sigmoid  Notch;  b,  Coronoid  Process;  c,  Condyle;  d,  Neck  of  Condyle;  e,  Ramus; 
/,  External  Oblique  Line;  g,  Angle;  h,  Mental  Foramen;  i,  Mental  Protuberance;  j,  Body. 

sidered  as  one  of  its  appendicular  elements.  It  is  the  heaviest  and 
strongest  bone  of  the  head,  gives  support  to  the  sixteen  lower  teeth,  and 
serves  as  a  framework  for  the  lower  half  or  floor  of  the  mouth.  It  is 
situated  at  the  lower  extremity  of  the  face,  and  immediately  below  the 
superior  maxillary  and  malar  bones,  while  its  posterior  extremity  rests 


INFERIOR    MAXILLARY    BONE  55 

against  the  glenoid  fossa  of  the  temporal  bone,  forming  a  movable  articu- 
lation with  this  cavity.  In  general,  the  bone  is  symmetric  in  outline, 
and  presents  for  examination  a  horizontal  portion,  or  body,  and  two 
vertical  portions,  or  rami,  which  in  the  adult  are  almost  perpendicular  to, 
or  at  right  angles  with,  the  body  of  the  bone. 

The  body,  or  horizontal  portion,  consists  of  two  identical  halves, 
which  meet  at  the  median  line  and  form  a  slight  vertical  ridge,  the  sym- 
physis. This  line  indicates  the  point  of  union  between  the  two  lateral 
halves,  which  at  birth  are  usually  separated,  but  soon  after  this  period 
become  firmly  united.  Each  lateral  half  of  the  body  presents  two  sur- 
faces— an  external  and  an  internal;  and  two  borders — a  superior  and  an 
inferior. 

The  external  or  facial  surface  (Fig.  20)  is  smooth  and  convex, 
and  furnishes  a  number  of  points  for  examination.  Beginning  at  the 
median  line,  the  symphysis  ends  inferiorly  in  a  prominent  triangular 
surface — the  mental  protuberance,  or  chin. 

The  Incisive  Fossa. — Passing  backward  from  the  symphysis,  and 
immediately  above  the  triangular  ridge  which  forms  the  mental  process, 
is  a  decided  but  shallow  depression — the  incisive  fossa.  This  fossa 
gives  origin  to  one  of  the  elevator  muscles  of  the  chin — the  levator  menti. 
Slightly  posterior  to  and  below  this  fossa,  on  a  line  corresponding  to  the 
position  of  the  cuspid  tooth,  is  an  oblong  depression  for  the  origin  of  the 
depressor  muscle  of  the  lower  lip — depressor  labii  inferioris. 

The  External  Oblique  Line. — Extending  obliquely  across  the  facial 
surface  from  the  mental  process  to  the  base  of  the  vertical  portion  of  the 
bone,  and  continuous  with  its  anterior  margin,  is  a  well-defined  ridge — 
the  external  oblique  line.  Xear  the  center  of  this  ridge,  or  below  the 
position  occupied  by  the  bicuspid  and  first  molar  teeth,  is  the  point  of 
attachment  of  the  depressor  muscle  of  the  angle  of  the  mouth — the  de- 
pressor anguli  oris.  Somewhat  anterior  to  and  above  this  point  is  the 
origin  of  the  depressor  muscle  of  the  lower  lip — the  depressor  labii  inferi- 
oris. Between  the  line  of  origin  of  the  depressor  anguli  oris  and  the  in- 
ferior border  of  the  bone  is  a  roughened  surface  for  the  attachment  of 
the  platysma  myoides  muscle.  This  roughened  surface  divides  the  body 
of  the  bone  into  an  upper  and  a  lower  portion.  That  portion  above  is 
known  as  the  alveolar  or  mucous  portion,  while  that  below  is  called  the 
basilar  or  non-mucous  portion.  The  attachment  of  the  platysma  myoides 
muscle  at  this  point  marks  the  lower  boundary  or  floor  of  the  mouth.  The 
superior  or  alveolar  portion  of  the  bone  is  within  the  cavity  of  the  mouth, 
and  is  covered  with  mucous  membrane  and  mucoperiosteum;  while  the 


56  ANATOMY 

inferior  or  basilar  portion  is  outside  and  below  the  cavity,  and  is  covered 
with  periosteum  similar  to  other  bones. 

The  Mental,  or  Anterior  Dental  Foramen. — Midway  between  the 
superior  and  inferior  border  of  the  body,  and  usually  below  the  second 
bicuspid  tooth,  is  a  large  foramen — the  mental  or  anterior  dental  fora- 
men— giving  passage  to  the  mental  branches  of  the  inferior  dental  nerve 
and  accompanying  blood-vessels.  The  position  of  this  foramen  is  not 
constant,  but,  as  previously  stated,  it  is  usually  below  the  second  bicuspid, 
or  between  this  point  and  the  first  bicuspid.  The  buccinator  muscle, 
which  forms  a  large  portion  of  the  lateral  wall  of  the  mouth,  has  its  origin 
from  the  facial  surface  of  the  mandible,  being  attached  to  the  alveolar 
portion  immediately  below  the  molar  teeth. 

The  Internal  Surface  of  the  Body  of  the  Bone  (Fig.  21). — The 
median  line  is  marked  by  a  slight  vertical  depression,  representing  the  line 
of  union,  and  corresponding  to  the  symphysis  externally. 

The  Mylohyoid,  or  Internal  Oblique  Ridge. — The  internal  surface  is 
divided  into  two  portions  by  a  well-defined  ridge — the  mylohyoid,  or 
internal  oblique  ridge.  It  occupies  a  position  closely  corresponding  to  the 
external  oblique  ridge  on  the  facial  surface.  Beginning  near  the  base 
of  the  bone  at  the  median  line,  it  passes  backward  and  upward,  increasing 
in  prominence  until  the  base  of  the  vertical  portion  of  the  bone  is  reached, 
into  which  it  gradually  disappears.  This  ridge  gives  origin  to  the  mylo- 
hyoid muscle,  which  forms  the  central  portion  of  the  floor  of  the  mouth. 
In  correspondence  to  the  facial  surface  of  the  bone,  the  attachment  of  the 
mylohyoideus  muscle  forms  the  dividing  line  between  the  mucous  mem- 
brane and  mucoperiosteum  covering  the  upper  portion  of  the  body  of 
the  bone,  and  the  periosteum  covering  the  inferior  portion. 

The  Genial  Tubercles. — Near  the  lower  third,  at  the  median  line,  is  a 
roughened  eminence — the  genial  tubercles.  Taken  collectively,  these  are 
in  two  pairs — a  superior  and  an  inferior.  The  superior  pair  (usually  the 
largest)  give  origin  to  the  geniohyoglossus  muscle,  and  the  lower  pair  to 
the  geniohyoid  muscle. 

The  Sublingual  Fossa. — By  the  side  of  the  genial  tubercles,  and 
above  the  mylohyoid  ridge,  is  a  shallow,  smooth  depression — the  sub- 
lingual fossa.  One  of  the  salivary  glands — the  sublingual — is  partially 
supported  in  this  fossa. 

The  Digastric  Fossa. — Below  the  mylohyoid-  ridge,  and  near  the 
median  line,  is  a  slight  depression — the  digastric  fossa — which  affords 
attachment  for  the  digastric  muscle. 

The  Submaxillary  Fossa. — In  the  center  of  the  internal  surface,  ex- 


INFERIOR    MAXILLARY    BONE 


57 


tending  from  before  backward,  between  the  mylohyoid  ridge  and  the 
lower  border  of  the  bone,  is  an  oblong  depression — the  submaxillary 
fossa.  In  this  fossa  rests  another  of  the  salivary  glands — the  maxillary. 
The  Superior  or  Alveolar  Border. — This  border  extends  from 
the  junction  of  the  body,  with  the  vertical  plate  on  one  side,  to  the  corre- 
sponding point  on  the  other.  The  construction  of  this  border  is  similar 
to  the  alveolar  border  of  the  superior  maxilla.     At  the  anterior  portion 


Fig.  21. — The  Mandible  or  Inferior  Maxilla.  Right  side,  Internal  Surface,  a,  Coronoid 
Process;  b.  Sigmoid  Notch;  c,  Cancellated  Tissue;  d.  Condyle;  e.  Ramus;/,  Inferior  Dental 
or  Mandibular  Foramen;  g,  Angle;  h,  Body;  i,  Internal  Oblique  Line. 

it  is  narrow,  but  gradually  increases  in  width  as  it  proceeds  backward — 
in  some  instances  following  the  line  of  the  body  of  the  bone;  in  others, 
inclining  inward,  or  to  the  lingual.  Each  lateral  half  is  marked  by  eight 
sockets,  for  the  accommodation  of  the  sixteen  lower  teeth.  They  are 
smaller  in  proportion  than  the  alveolar  sockets  in  the  superior  maxilla. 
The  socket  nearest  the  median  line  receives  the  central  incisor  tooth, 
and  is  the  smallest  of  the  number.  It  has  an  average  depth  of  7/16  of 
an  inch,  is  conic  from  above  downward,  oblong  in  transverse  section, 
with  its  lateral  walls  flattened.  The  second  socket  gives  support  to 
the   lateral  incisor;   it  is  a  trifle  larger  than 'the  central  incisor  socket, 


58  ANATOMY 

but  in  other  respects  is  quite  similar.  The  socket  for  the  cuspid 
is  situated  at  the  anterior  angle  of  this  border,  and  is  much  larger 
and  deeper  than  the  incisor  sockets.  It  has  an  average  depth  of  9/16 
of  an  inch;  its  lateral  walls  are  compressed,  and  sometimes  slightly 
concave.  In  transverse  section  the  labial  wall  forms  a  larger  curve  than 
the  internal  or  lingual  wall.  Passing  backward,  the  next  two  sockets  are 
for  the  support  of  the  bicuspids;  they  are  circular  in  outline,  with  an  aver- 
age depth  of  1/2  of  an  inch.  The  cavity  for  the  first  bicuspid  is  usually 
a  little  larger  than  that  for  the  second.  In  rare  instances  one  or  the  other 
of  these  sockets  will  be  divided  for  the  accommodation  of  two  roots. 
The  sockets  for  the  first  and  second  molars  present  a  circular  outline 
upon  their  free  margins,  but  below  they  divide  into  two  flattened,  cone- 
shaped  cavities — one  anterior  and  one  posterior.  The  flattened  sides  of 
these  cavities  are  concave  in  the  center,  and  at  their  lower  third  curve 
backward.  The  average  depth  of  these  sockets  is  1/2  of  an  inch.  The 
socket  for  the  third  molar,  like  its  superior  fellow,  is  variable  both  in  form 
and  position,  frequently  being  crowded  inside  or  outside  of  the  tooth-line. 
In  some  instances  it  is  divided  into  two  or  more  compartments.  The 
average  depth  is  not  over  3/8  of  an  inch. 

The  alveolar  process,  which  composes  the  superior  border  of  the 
body  of  the  mandible,  differs  from  the  same  process  in  the  superior  max- 
illa in  one  very  important  particular:  instead  of  the  outer  plate  being 
thin  and  frail,  it  is  equally  as  heavy  as  the  inner  or  lingual  plate.  When 
the  tooth-line  is  inclined  inward  from  the  body  of  the  bone,  the  posterior 
outer  wall  is  much  heavier  than  the  interior  or  lingual  wall. 

The  Inferior  Border  of  the  Body  of  the  Bone. — This  border  extends 
from  a  slight  depression,  to  be  observed  at  the  point  of  union  between 
the  body  and  ramus,  to  the  corresponding  point  upon  the  opposite  side. 
It  is  strong,  rounded,  and  compact,  and  gives  to  the  bone  the  greatest 
portion  of  its  strength.  Near  its  junction  with  the  ramus  is  the  facial 
notch,  so  named  from  the  facial  artery  passing  over  this  point. 

The  Ramus,  or  Vertical  Portion  of  the  Bone. — This  vertical 
plate  is  quadrilateral  in  outline,  and  presents  two  surfaces — extrenal 
and  internal;  four  borders — superior,  inferior,  anterior,  and  posterior; 
and  two  processes — the  condyloid  and  the  coronoid. 

The  external  surface  is  flat  and  smooth.  Near  the  center  it  is  slightly 
concave  and  roughened  for  the  attachment  of  one  of  the  muscles  of  mas- 
tication— the  masseter. 

The  internal  surface  presents  near  the  center  an  oblong  opening — 
the  inferior  dental  or  mandibular  foramen — leading  into  the  inferior  dental 


INFERIOR    MAXILLARY    BONE  59 

or  mandibular  canal.  Surrounding  this  foramen,  on  its  posterior  inter- 
nal margin,  is  the  mandibular  spine,  to  which  is  attached  the  spheno- 
mandibular  ligament.  Running  obliquely  downward  from  the  base  of 
the  foramen,  and  beneath  the  spine,  is  a  decided  groove — the  mylohyoid 
groove — which  accommodates  the  mylohyoid  nerve,  artery,  and  vein, 
which  pass  forward  to  supply  the  floor  of  the  mouth.  Below  and  behind 
this  groove  the  surface  is  roughened  for  the  attachment  of  another  muscle 
of  mastication— the  internal  pterygoid. 

The  Inferior  Dental  or  Mandibular  Canal. — Beginning  at  the  foramen 
of  the  same  name,  this  canal  enters  the  body  of  the  bone,  passes  downward 
and  forward  horizontally,  until  it  finds  an  exit  at  the  mental  foramen. 
This  canal  lies  immediately  below  the  alveolar  sockets,  and  from  it  are 
given  off  smaller  canals  which  open  into  the  tooth-sockets  through 
minute  foramina.  Near  the  mental  foramen  the  canal  divides  into  a 
number  of  smaller  ones,  which  pass  forward  through  the  substance  of  the 
bone  to  the  sockets  of  the  cuspid  and  incisor  teeth. 

The  Superior  Border  of  the  Ramus. — This  border  is  crescent-shaped, 
and  is  otherwise  known  as  the  sigmoid  notch.  Arising  from  its  anterior 
portion  is  a  flattened,  cone-shaped  process — the  coronoid  process.  On 
its  posterior  portion  is  a  rounded  or  oblong  eminence — the  condyloid 
process.  The  concave  or  crescent-shaped  margin  of  this  border  is  thin 
and  smooth  in  front,  becoming  wider  and  heavier  as  it  approaches  the 
condyle. 

The  Coronoid  Process. — The  anterior  margin  of  this  process,  being 
a  continuation  of  the  external  oblique  line,  is  heavier  at  the  base  than  at 
the  apex.  The  outer  surface  is  smooth,  and  affords  attachment  to  the 
masseter,  and  a  few  fibers  of  the  temporal  muscle.  The  internal  surface 
is  marked  by  a  vertical  ridge,  which  passes  downward,  increasing  in  size, 
and  finally  joining  the  internal  oblique  line  at  a  point  posterior  to  the 
third  molar.  The  surface  anterior  to  this  ridge  is  grooved,  and  gives 
attachment  to  a  part  of  the  temporal  muscle  above,  and  the  buccinator 
muscle  below.  The  surface  posterior  to  this  ridge  affords  attachment 
for  the  greater  part  of  the  temporal  muscle.  The  posterior  border  of 
this  surface  is  thin,  and  forms  the  anterior  margin  of  the  sigmoid  notch. 

The  Condyloid  Process. — This  may  be  described  as  the  expanded 
extremity  of  the  posterior  border  of  the  ramus,  and  is  quite  variable  in 
form  (see  Occlusion  of  the  Teeth).  It  is  divided  into  a  superior  or 
articular  portion,  and  an  inferior  portion,  or  neck. 

The  articular  portion  of  the  condyle  is  more  or  less  oblong,  and  is 
convex  above,  fitting  into  the  glenoid  fossa  of  the  temporal  bone,  and 


6o  ANATOMY 

forming,  with  the  intcrarticular  cartilage  which  lies  between  the  two  sur- 
faces, the  temporomaxillary  articulation. 

The  neck  is  that  constricted  portion  immediately  below  the  articular 
surface.  It  is  flattened  in  front  and  presents  a  pit — the  pterygoid  fossa — 
to  which  a  portion  of  the  pterygoid  muscle  is  attached.  Immediately 
below  the  point  of  junction  between  the  neck  and  the  articular  surface 
externally  is  the  condyloid  tubercle,  to  which  is  attached  the  external 
lateral  ligament. 

The  Inferior  Border  of  the  Ramus. — This  border  is  thick,  rounded,  and 
continuous  with  the  lower  border  of  the  body  of  the  bone.  At  the  point 
of  junction  between  this  and  the  posterior  border  is  the  angle  of  the  jaw. 
The  angle  has  a  slight  outward  inclination,  and  is  roughened  for  the 
attachment  of  a  part  of  the  superficial  portion  of  the  masseter  muscle. 

The  anterior  border  has  been  described  in  connection  with  the  coro- 
noid  process. 

The  posterior  border  is  smooth  and  rounded  on  its  upper  half,  the 
lower  half  being  roughened  for  the  attachment  of  the  stylomaxillary 
ligament. 

Attachment  of  Muscles. — The  following  muscles  are  attached 
to  the  mandible: 

Buccinator,  Superior  constrictor  of  pharynx, 

Depressor  labii  inferioris,  Masseter, 

Depressor  anguli  inferioris,  Orbicularis  oris, 

Levator  menti,  Internal  and  external  pterygoid, 

Geniohyoglossus,  Geniohyoid, 

Platysma  myoides,  Mylohyoid, 

Digastric,  Temporal. 

Development.* — On  account  of  its  early  functional  activity,  the 
mandible  is  among  the  first  bones  to  ossify.  Development  takes  place 
from  six  centers  for  each  lateral  half,  the  nuclei  being  deposited  as  early 
as  the  sixth  or  eighth  fetal  week,  and  after  their  establishment  the  develop- 
mental process  takes  place  very  rapidly.  The  six  centers  of  ossification 
are  principally  named  according  to  their  position.  The  early  preparation 
for  the  development  of  the  bone  is  found  in  the  appearance  of  what  is 
known  as  the  mandibular  plates,  which  are  thrown  out  from  the  sides 
of  the  cranial  base,  and  finally  unite  at  the  median  line.  Not  long  after 
this  period  a  cartilaginous  band — Meckel's  cartilage — is  developed  in  the 
substance  of  the  mandibular  plates,  and  is  it  about  this  cartilaginous 

*See  "Development  of  the  Teeth." 


INFERIOR    MAXILLARY    BONE 


6l 


framework  that  ossification  first  takes  place.  The  various  centers  are 
distributed  along  the  line  of  Meckel's  cartilage,  and  are  named  as  follows: 
Mental,  dentinary,  coronoid,  condyloid,  angular,  and  splenic.  The 
mental  center  provides  for  the  development  of  that  portion  of  the  bone 
between  the  median  line  and  the  mental  foramen.     The  dentinary  center 


Childhood 


Adah 


Senile 


Fig.  22. — Chart  Showing  the  Evolution  and  Degeneracy  of  the  Mandible. 


forms  the  lower  border  and  outer  plate,  and  provides  for  the  establish- 
ment of  the  crypts  inclosing  the  developing  tooth-follicles. 

The  coronoid  and  condyloid  centers  are  both  instrumental  in  con- 
structing these  processes,  and  the  angular  center  provides  for  the  angle  of 
the  bone.  The  splenic  center  is  somewhat  later  in  making  its  appearance, 
and  from  it  the  inner  plate  of  the  mandible  is  formed,  the  line  of  union 


62  ANATOMY 

between  it  and  the  dentinary  center  being  indicated  by  the  mylohyoid 
groove.  While,  as  above  stated,  most  of  the  centers  of  development  are 
along  or  near  the  line  of  Meckel's  cartilage,  the  condyloid  and  coronoid 
processes  are  developed  from  other  cartilage.  Soon  after  birth  the  two 
lateral  halves  of  the  mandible  begin  to  coalesce  at  the  median  line,  this 
union  taking  place  from  below  upward;  and  by  the  eighth  or  tenth  month 
union  is  complete  and  the  individual  bone  is  established.  The  inferior 
maxilla  is  subject  to  a  continuous  change  in  form,  not  only  in  regard  to 
its  general  contour,  but  also  accommodating  itself  to  the  movements  and 
growth  of  the  teeth,  the  former  taking  place  at  or  about  the  angle,  while 
the  latter  occurs  in  the  alveolar  portion  of  the  bone. 

Figure  22  represents  the  changes  which  take  place  in  the  angle  of 
the  mandible  from  youth  to  old  age.  It  will  be  observed  that  the  angle 
formed  in  the  adult  bone,  with  the  teeth  in  position,  is  almost  a  right 
angle;  and  that  in  youth,  with  the  deciduous  teeth  in  the  alveoli,  the  angle 
is  much  more  obtuse,  which  condition  is  again  approached  in  old  age. 

THE  HYOID  BONE. 

This  is  a  U-shaped  bone  placed  in  the  upper  part  of  the  neck  at  the 
median  line  near  the  base  of  the  tongue.  It  has  no  bony  connection  with 
other  bones;  it  is  classed  as  a  floating  bone.  It  is  made  up  of  a  body 
and  four  processes. 

The  body,  or  central  portion,  is  quadrilateral  in  outline,  somewhat  ob- 
long from  side  to  side,  with  its  anterior  aspect  convex,  and  presents  a 
longitudinal  ridge  which  divides  it  into  a  superior  and  an  inferior  portion. 
It  is  also  usually  divided  at  the  median  line  by  a  slight  vertical  ridge. 
At  the  point  of  junction  between  the  longitudinal  and  vertical  ridges  a 
slight  tubercle  is  formed.  The  anterior  surface  is  given  up  to  the  attach- 
ment of  muscles.  The  posterior  surface  of  the  body  of  the  bone  is  con- 
cave and  smooth,  and  is  directed  backward  and  downward.  The  superior 
border  gives  attachment  to  the  thyrohyoid  membrane,  while  the  inferior 
border,  which  is  somewhat  thicker,  gives  attachment  to  the  sternohyoid 
and  thyrohyoid  muscles. 

The  processes  known  as  the  greater  and  lesser  cornua  are  four  in  num- 
ber, one  of  each  kind  on  either  side. 

The  greater  cornua  project  backward  and  upward,  and  their  lower 
borders  and  anterior  surfaces  are  occupied  with  muscles.  The  thyro- 
hyoid ligament  is  attached  to  the  posterior  terminal  corner. 

The  lesser  cornua  are  short  conical  pieces  of  bone,  and  project  upward 


THE   HYOID    BONE  63 

and  backward  from  the  extremities  of  the  body  of  the  bone.  They 
give  attachment  to  the  stylohyoid  ligaments. 

Development. — Ossification  takes  place  from  five  centers,  one  for 
the  body  of  the  bone  and  one  for  each  cornua. 

Attachment  of  Muscles. — 

.  Geniohyoglossus,  Sternohyoid, 

Geniohyoid,  Lingualis, 

Thyrohyoid,  Omohyoid, 

Mylohyoid,  Digastric, 

Hyoglossus,  Middle  constrictor. 

The  thyrohyoid  ligament,  as  well  as  the  stylohyoid,  and  the  thyrohyoid 
membrane  are  also  attached  to  this  bone. 


CHAPTER  IV. 

The    Temporomandibular    Articulation.     The    Muscles     of 

Mastication. 

TEMPOROMANDIBULAR  ARTICULATION. 

Although  external  to  the  cavity  of  the  mouth,  this  articulation  is  so 
closely  associated  with  the  masticatory  function  that  it  seems  important 
that  a  brief  description  of  its  construction  and  action  should  be  given. 
It  receives  its  name  from  the  two  bones  which  enter  into  its  formation— 
the  temporal  and  the  mandible,  or  inferior  maxillary. 


Condyle 


Ramus  of 
Mandible 


Glenoid 
•Fossa 


Coronoid 
Process 


Fig.  23. — Temporomandibular  Articulation. 

This  joint  is  the  seat  of  motion  in  the  mandible,  and  entering  into  its 
construction  are  bones,  ligaments,  cartilage,  and  synovial  membrane, 
these  being  the  tissues  essential  to  all  diarthrodial  or  movable  articulations. 
The  various  movable  joints  of  the  body  are  classified  according  to  the 

64 


TEMPOROMANDIBULAR   ARTICULATION  65 

nature  of  the  movement,  and  correspond  to  the  mechanical  actions  known 
as  hinge  joint,  ball-and-socket  joint,  gliding  joint,  pulley  joint,  etc.  The 
temporomandibular  joint  is  of  the  diarthrodial  class,  and  the  movements 
which  it  possesses  are  a  combination  of  the  gliding  movement  (arthrodia) 
and  of  the  hinge  movement  (ginglymus).  The  osseous  parts  entering 
into  the  formation  of  the  joint  are  the  anterior  portion  of  the  glenoid 
fossa  of  the  temporal  bone  and  the  condyloid  process  of  the  mandible 

(Fig-  23). 

The  glenoid  fossa  may  be  described  as  an  oblong  cavity,  with  its 
base  directed  upward,  being  bounded  anteriorly  by  a  heavy  bone  ridge 
(the  anterior  root  of  the  zygoma),  posteriorly  by  an  irregular,  flattened 
portion  of  the  bone  (the  tympanic  plate  of  the  petrous  portion),  internally 
by  a  union  of  the  anterior  and  posterior  boundaries,  and  externally  by  the 
middle  root  of  the  zygoma.  The  floor  of  the  fossa  is  traversed  by  a  well- 
marked  fissure — the  glenoid  fissure  (fissure  of  Glaserius) — which  divides 
the  fossa  into  two  portions,  an  anterior  and  a  posterior.  The  anterior 
half  is  deeper  and  more  concave  than  the  posterior,  and  is  the  articulating 
portion,  being  occupied  by  the  condyle,  while  the  posterior  half  gives 
lodgment  to  the  parotid  gland. 

The  condyloid  process  of  the  mandible  having  been  described 
with  that  bone,  in  this  connection  reference  will  be  made  to  the  variety 
of  forms  which  it  presents,  and  the  influence  which  it  exerts  over  the 
nature  of  the  tooth  occlusion.  This  process,  when  narrow  and  oblong 
(Fig.  23),  closely  resembles  the  ginglymus,  or  hinge  joint,  and  will  be 
accompanied  by  teeth  presenting  deep,  penetrating  cusps,  forming  a 
positive  and  well-locked  occlusion  (Fig.  24,  B),  with  little  or  no  lateral 
motion.  If  the  condyle  presents  the  appearance  shown  in  figure  25, 
which  resembles  the  enarthrodia,  or  ball-and-socket  joint  (although  it 
cannot  be  considered  as  such),  the  teeth  associated  with  such  a  formation 
will  be  provided  with  short,  rounding  cusps,  and  the  occlusion  will  be 
loose  and  wandering  (Fig.  24,  A).  This  difference  in  the  form  of  the  con- 
dyle will  be  accompanied  by  a  corresponding  variation  in  the  concavity 
of  the  glenoid  fossa.  Not  only  does  the  osseous  structure  in  the  joint 
partake  of  individual  characteristics,  but  likewise  the  muscles  and  liga- 
ments; their  functions  being  to  operate  the  articulation,  they  are  developed 
in  accordance  with  the  action  required  of  them,  which  action  is,  in  a 
measure,  dependent  upon  the  conditions  existing  in  the  mouth. 

Both  the  condyle  and  the  glenoid  fossa  are  covered  with  articular  car- 
tilage. In  the  latter  this  membrane  extends  over  its  anterior  border,  to 
facilitate  the  play  of  the  joint.  The  condyle  is  held  in  position  in  the  fossa 
5 


66 


ANATOMY 


by  three  ligaments — the  capsular,  the  sphenomaxillary,  and  stylomaxillary. 
The  capsular  ligament  is  divided  into  four  portions — anterior  and  posterior, 
external  and  internal.     The  anterior  portion  consists  of  a  few  fibers 


Fig.  25. 

connected  with  the  anterior  margin  of  the  fibrocartilage,  attached  below 
to  the  anterior  margin  of  the  condyle  and  above  to  the  front  of  the  glenoid 
ridge.     The  posterior  portion  is  attached  above  just  in  front  of  the  glenoid 


TEMPOROMANDIBULAR   ARTICULATION 


67 


fissure,  and  is  inserted  into  the  posterior  margin  of  the  ramus  of  the 
maxilla  just  below  the  neck  of  the  condyle.  The  external  portion, 
otherwise  known  as  the  external  ligament,  is  the  strongest  portion  of  the 
capsular  ligament.  It  has  a  broad  attachment  above  to  the  zygoma, 
from  which  point  it  passes  downward  and  backward,  and  is  inserted  into 
the  outer  side  of  the  neck  of  the  condyle.     The  internal  portion,  or  short 


Styloid  Process    Capsular  Ligament 


Internal  Lateral  Ligament 


Stylohyoid  Ligament 

Stylomaxillary  Ligament 
Fig.  26. — Temporomaxillary  Articulation — -Internal  Veiw.     (Deaver.) 

internal  lateral  ligament,  is  composed  of  well-defined  fibers,  having  a 
broad  attachment  above  to  the  inner  edge  of  the  glenoid  fossa  and  to  the 
alar  spine  of  the  sphenoid  bone;  below  it  is  inserted  into  the  inner  side  of 
the  neck  of  the  condyle. 

The  sphenomaxillary,  or  long  internal  lateral  ligament,  is  a  thin, 
loose  band,  situated  some  distance  from  the  joint  proper,  and,  as  its  name 
implies,  has  its  attachment  above  to  the  alar  spine  of  the  sphenoid  bone, 


68 


ANATOMY 


and  also  to  that  portion  of  the  temporal  bone  contiguous  to  it.  It  passes 
downward  and  forward,  and  is  inserted  into  the  mandibular  spine  of  the 
maxilla. 

The  stylomaxillary  ligament  extends,  from  the  styloid  process  of  the 
temporal  bone,  downward  and  forward,  to  be  inserted  into  the  poste- 


External  Lateral  Ligament 


Capsular  Ligament 


Styloid  Process 
Stylohyoid  Ligament 
Stylomaxillary  Ligament 
Fig.  27. — Temporomaxillary  Articulation — External  View.     (Deaver.) 


rior  border  of  the  ramus  of  the  inferior  maxilla,  at  a  point  between  the 
masseter  and  internal  pterygoid  muscles. 

The  inter  articular  fibrocartilage  is  an  oval  sheet  placed  between  the 
two  articulating  surfaces.  It  is  thinnest  at  the  center  and  becomes  thicker 
as  the  margins  of  the  fossa  are  approached,  at  which  point  it  is  connected 
with  the  fibers  of  the  capsular  ligament.     Being  placed  immediately  be- 


TEMPOROMANDIBULAR  ARTICULATION 


69 


tween  the  two  articular  surfaces  it  divides  the  joint  into  two  separate 
synovial  cavities.  Each  of  these  synovial  cavities  is  occupied  by  a  syno- 
vial membrane,  that  occupying  the  upper  compartment  being  the  largest, 
and  passes  from  the  margins  of  the  glenoid  fossa  above  to  the  upper 
surface  of  the  interarticular  cartilage  below.  The  membrane  which 
occupies  the  lower  cavity  is  smaller,  and  passes  from  the  under  surface 
of  the  interarticular  cartilage  above  to  the  margins  of  the  condyle  below. 
The  blood-supply  to  this  articulation  is  derived  from  the  temporal,  middle 
meningeal,  and  ascending  pharyngeal  arteries. 

The  nerves  are  derived  from  the  masseteric  and  auriculotemporal. 


Fig.  28. — Showing  Variation  in  the  Shape  of  the  Condyles. 


The  movements  of  this  articulation  present  as  great  a  range  as  any 
other  joint  in  the  human  body.  While  the  chief  movement  is  of  the  gingly- 
moid  or  hinge  character,  brought  into  play  in  simple  depression  and 
elevation  of  the  mandible,  it  also  has  the  power  of  extension  and  retrac- 
tion, may  be  rotated  from  side  to  side,  together  with  all  the  motions 
intermediate  between  these.  When  the  mandible  is  depressed,  the  con- 
dyle moves  on  the  fibrocartilage,  and  at  the  same  time  glides  forward  and 
slightly  downward  until  it  rests  on  the  anterior  border  of  the  glenoid 
fossa; — this  movement  does  not  extend  sufficiently  to  allow  the  condyle 
to  rest  upon  the  extreme  summit  of  the  border,  except  in  cases  of  excessive 
movement,  as  in  yawning,  when  the  condyle  may  glide  over  the  summit 


7o 


ANATOMY 


and  the  joint  become  disarticulated.  When  the  mandible  is  elevated,  the 
condyle  slides  backward  and  upward,  and  at  the  same  time  the  fibro- 
cartilage,  which  has  extended  with  it,  also  retracts  until  the  condyle  is 
settled  in  the  fossa.  The  movement  of  extension  and  retraction  is  by  a 
horizontal  gliding  action,  by  which  the  mandible  is  thrust  forward  and 

Temporal  Muscle 


Buccinator  Muscles 

Masseteric  Nerve 
I  .     .  Masseteric  Artery 
Facial  Artery 
Facial  Vein 


Superficial  Temporal 

Artery 
Facial  Nerve 
Masseter  Muscle 

Platysma  Myoides 
Muscle 


Fig.   29. — Temporal  Muscle.     (Deaver.) 

drawn  back  again.  In  this  movement,  as  well  as  in  the  one  previously 
described,  both  condyles  are  similarly  and  simultaneously  engaged.  The 
lateral  or  triturating  movement  is  made  in  an  oblique  direction.  This 
consists  in  a  rotation  of  the  condyles  within  the  fossae,  the  cartilage  gliding 
obliquely  forward  and  outward  on  one  side,  and  backward  and  inward 
on  the  other,  this  action  taking  place  alternately.     This  movement  is 


TEMPOROMANDIBULAR   ARTICULATION  7 1 

more  or  less  developed  in  accordance  with  the  nature  of  the  occlusion, 
being  favored  by  those  teeth  possessing  but  little  cusp  formation,  with  a 
consequent  loose  and  wandering  occlusion;  while  in  that  type  of  tooth 
associated  with  a  long  overbite  and  deep  penetrating  cusps,  forming  a 
firm  and  well-locked  occlusion,  this  movement  will  be  but  little  developed. 
If  this  movement  be  employed  to  throw  the  symphysis  to  one  bide  and 
back  again,  and  not  from  side  to  side,  the  condyle  of  that  side  rotates  in 
the  glenoid  fossa,  while  the  condyle  of  the  opposite  side  is  drawn  forward 
and  inward. 

The  Muscles  of  Mastication. 

Occupying  the  back  part  of  the  side  of  the  face,  and  forming  an  in- 
dependent group,  are  four  muscles,  usually  classed  as  the  muscles  of 
mastication.  While  this  is  true  to  a  great  degree,  they  are  not  the  only 
muscles  brought  into  action  during  this  process.  They  are  the  masseter, 
temporal,  internal  pterygoid,  and  external  pterygoid.  The  masseter, 
temporal,  and  internal  pterygoid  lift  or  close  the  lower  jaw,  the  principal 
function  of  the  external  pterygoid  being  to  extend  the  lower  jaw  so  that 
the  lower  teeth  pass  beyond  the  upper.  The  muscles  which  open  the 
jaws,  such  as  happens  when  the  head  is  thrown  backward,  are  the  muscles 
of  the  neck. 

The  masseter  muscle  is  made  up  of  a  strong  quadrate  sheet,  con- 
sisting of  two  distinct  layers  extending  from  the  zygomatic  arch  to  the 
mandible.  The  layers  of  which  the  muscle  is  composed  differ  somewhat 
in  size  as  well  as  in  the  directions  which  they  take. 

Origin. — The  superficial  layer,  much  the  stronger  and  larger  of  the 
two,  arises  from  the  lower  border  of  the  malar  bone,  and  from  the  anterior 
two-thirds  of  the  zygomatic  arch.  The  deep  layer  arises  from  the  posterior 
third  of  the  lower  border  and  from  nearly  all  the  internal  surface  of  the 
zygomatic  arch. 

Insertion. — After  passing  downward  and  backward  it  is  inserted  into 
the  outer  surface  of  the  ramus  of  the  mandible.  The  deep  layer,  passing 
downward  and  slightly  forward,  mingles  with  some  of  the  fibers  of  the 
superficial  portion,  and  is  finally  inserted  into  the  upper  half  of  the  ramus 
of  the  mandible. 

Action. — To  draw  slightly  forward,  and  by  its  superficial  layer  to 
close  the  jaw.  "In  closing  the  jaw  it  acts  with  less  mechanical  disad- 
vantage than  is  usual  with  muscles.  When  the  pressure  to  be  overcome 
is  exerted  upon  the  back  teeth,  the  arm  of  the  lever  upon  which  the  power 
acts  is  almost  as  Ions;  as  that  which  intervenes  between  these  teeth  and  the 


72 


ANATOMY 


fulcrum.  This  fulcrum  is  not  at  the  temporomaxillary  joint,  but  at  a 
point  below  the  neck  of  the  mandible,  corresponding  very  nearly  to  the 
lower  attachment  of  the  internal  lateral  ligament.  Moreover,  the  result- 
ant force  of  the  muscle  acting,  as  it  does,  upward  and  forward,  is  perpen- 
dicular to  the  lever,  which  may  roughly  be  described  as  a  bar  extending 
downward  and  forward  from  the  neck  of  the  mandible  to  the  point  of  the 
chin."     (Morris.) 


Interarticular  flbro-cartilage 


External  pterygoid 


Internal  pterygoid 


Fig.  30. — The  Pterygoid  Muscles.     (Morris.) 

Relations. — It  is  covered  superficially  by  the  skin  and  fascia  of  the 
platysma  myoides,  by  the  risorius  and  the  masseter  fascia,  by  the  parotid 
gland  and  ducts,  facial  veins,  and  portions  of  the  facial  nerve.  Deeply,  the 
muscle  lies  in  contact  with  the  ramus  of  the  jaw  and  the  buccinator  muscle, 
being  separated  from  the  latter  by  a  layer  of  fat.  A  small  portion  of  the 
temporal  muscle  also  comes  in  relation  to  the  deeper  lying  portion. 

The  temporal  muscle  is  covered  by  a  strong  membrane,  the  temporal 
fascia,  which  arises  from  the  temporal  ridge,  and  is  inserted  into  both  the 
inner  and  outer  portions  of  the  upper  border  of  the  zygomatic  arch.  Near 
this  point  it  divides  into  two  distinct  layers,  one  passing  to  the  inner,  the 
other  to  the  outer  margins  of  the  zygoma.     Below  the  fascia  is  continuous 


TEMPOROMANDIBULAR   ARTICULATION  73 

with  the  masseteric  fascia.  Passing  downward  from  this  to  the  inferior 
borders  of  the  ramus  of  the  jaw,  it  envelops  the  masseter  muscle.  The 
muscle  itself  is  radiating  and  fan-shaped  in  form,  located  in  the  temporal 
fossa,  from  which  point  it  descends  to  the  coronoid  process  of  the 
mandible. 

Origin. — From  nearly  the  entire  surface  of  the  temporal  fossa,  from 
the  temporal  ridge,  from  the  entire  surface  of  the  temporal  fascia,  down 
to  its  lower  attachment  to  the  zygomatic  process. 

Insertion. — Into  the  coronoid  process  of  the  mandible. 

Action. — To  close  the  lower  jaw,  some  of  its  fibers  drawing  the  jaw 
backward  after  the  other  muscles  have  protruded  it. 

Relations. — Its  superficial  portion  is  covered  by  the  temporal  fascia 
which  separates  it  from  some  of  the  auricular  muscles;  branches  of  the 
facial  nerve,  the  auriculotemporal  nerve,  and  a  portion  of  the  epicranial 
aponeurosis.  The  temporal  fossa  and  the  external  pterygoid  muscles 
are  in  relation  with  it  deeply. 

The  internal  pterygoid  muscle  is  a  thick,  quadrilateral,  sheet- 
like muscle,  and  receives  its  name  from  its  origin  and  relative  position. 

Origin. — From  the  inner  surface  of  the  external  pterygoid  plate;  the 
tuberosity  of  the  palate  bone  and  a  small  portion  of  the  maxilla. 

Insertion.— Into  the  internal  surface  of  the  ramus  of  the  mandible  at 
its  lower  and  posterior  borders  and  extending  as  high  as  the  mandibular 
foramen  and  mylohyoid. 

Action. — To  close  the  jaw  and  at  the  same  time  draw  it  backward  and 
throw  it  toward  the  opposite  side.  "The  same  remarks  which  were 
made  with  respect  to  the  very  small  loss  of  mechanical  advantage  in  the 
masseter  muscle  apply  to  this  muscle.  When  closed  it  will  draw  the 
jaw  forward;  and  also  it  will  help  the  external  pterygoid  in  drawing  the 
ramus  of  its  own  side  toward  the  middle  line."     (Morris.) 

Relations. — Superficially,  the  internal  maxillary  vessels,  the  external 
pterygoid  muscle,  the  internal  lateral  ligament,  the  inferior  dental  and 
lingual  nerves.  Deeply,  the  submaxillary  glands;  the  tensor  palati  and 
superior  constrictor  of  the  pharynx,  as  well  as  the  stylohyoid  and  posterior 
border  of  the  digastric  muscles. 

The  external  pterygoid  muscle  is  composed  of  two  triangular  sheets, 
one  passing  in  a  horizontal  and  the  other  in  a  vertical  direction.  It  re- 
ceives its  name  from  its  attachment  to  the  pterygoid  process  of  the  sphenoid 
bone  as  well  as  its  relation  to  its  companion  muscle,  the  internal  pterygoid. 

Origin. — It  is  composed  of  two  distinct  heads,  an  upper  and  a  lower. 
The  upper  head  arises  from  the  greater  wing  of  the  sphenoid  bone,  from 


74  ANATOMY 

the  internal  pterygoid  ridge,  and  external  to  the  foramen  ovale  and  fora- 
men spinosum.  The  lower  head  arises  from  the  outer  surface  of  the  ex- 
ternal pterygoid  plate. 

Insertion. — The  upper  head  is  inserted  into  the  inter-articular  fibro- 
cartilage,  into  the  capsule  of  the  joint  as  well  as  the  neck  of  the  condyle. 
The  lower  head  is  inserted  into  the  neck  of  the  condyle. 

Action. — To  draw  the  condyle  and  inter- articular  fibrocartilage  for- 
ward and  inward.  "The  combination  of  these  two  movements  produces 
the  oblique  movement  of  the  lower  molar  teeth  of  one  side,  forward  and  in- 
ward with  respect  to  the  upper  molars  which  are  their  opponents.  It 
should  be  observed  also  that  this  inward  movement  of  one  side  is  the 
agent  by  which  the  ramus  of  the  opposite  side  is  moved  outward.  To 
assist  in  opening  the  mouth  by  depression  of  the  lower  jaw.  As  the 
transverse  axis  of  this  movement  passes  through  the  mandible  at  two 
points  situated  below  the  necks  of  the  rami,  it  follows  that  a  forward 
movement  of  the. condyles  and  necks  will  assist  in  the  backward  move- 
ment of  the  angles  and  body  which  accompanies  the  depression  of  the 
mandible."     (Morris.) 

Relations. — Superficially,  some  of  the  fibers  of  the  internal  pterygoid, 
the  temporal,  and  part  of  the  masseter  muscle.  Deeply,  the  internal 
pterygoid  muscle,  the  middle  meningeal  and  inferior  dental  vessels,  in- 
ternal maxillary  vessels,  and  masseteric  and  posterior  deep  temporal 
nerves  passing  behind  or  through  the  attachment  to  the  upper  head;  the 
inferior  dental  and  lingual  gustatory  nerves  beneath  the  lower  head. 


CHAPTER  V. 

A    General    Description    of   the   Teeth. — The    Permanent   Teeth 
Classification,    Surfaces,    Etc. — The    Roots    of    the    Teeth.— 
The  Dental  Arch. 


A  GENERAL  DESCRIPTION  OF  THE  TEETH. 

A  Tooth  (Fig.  31). — One  of  the  thirty-two  specialized  organs  for  the 
seizure  and  mastication  of  food,  placed  at  the  entrance  to  the  alimentary 
canal  (the  mouth).     The  typical  form  of  a  tooth  is  a  modified  cone  or 
combination  of  cones,  and  is  composed  of 
two  fundamental  parts — the  crown  and  the 
root  or  roots.     The  crown  is  that  part  which 
projects  beyond  the  gum  and  is  visible  in 
the  mouth;  while  the  root  is  that  part  which 
is  implanted  in  the  bone  and  covered  by 
the   gum.     Intervening  between  these  two 
extremities,  and  usually  occupying  a  por- 
tion of  each,  is  a  third  division — the  neck. 

Completely  covering  the  crown  of  a 
tooth  is  a  hard,  vitreous-like  substance, 
enamel;*  the  root  is  covered  by  a  hard, 
bone-like  substance,  cementum;*  while  the  Crown j 
interior  or  body  of  the  organ  is  composed 
of  a  hard  substance  closely  resembling  bone, 
the  dentin*  The  neck  of  a  tooth,  which 
serves  to  unite  the  crown  to  the  root,  and 
which  is  usually  formed  at  the  expense  of 
each,  is  covered  partly  by  enamel  and  partly 

by  cementum.  This  brief  description  in  the  singular  number  can  best  be 
continued  in  the  plural.  Teeth  are  classified  according  to  their  form,  which 
is  always  in  accordance  with  their  function,  into  simple  and  complex.  In 
the  simple  class  the  single  modified  cone  is  the  predominating  form,  the  free 
extremity  of  the  crown  serving  as  the  base  of  the  cone,  while  the  apex  is 
formed  bv  the  free  end  of  the  root.     Included  in  this  same  classification 


Aoe.- 


Root 


Neck 


Fig. 


*See  Tissues  of  the  Teeth,   Part   1 1 . 


75 


76  ANATOMY 

are  those  teeth  which  are  made  up  of  a  double  cone,  or  a  simple  cone  and 
an  inverted  cone  attached  to  each  other  at  a  common  base  (Fig.  31). 
The  purposes  for  which  such  teeth  are  adapted  are  those  of  grasping, 
incising,  and  tearing  and  they  are  so  arranged  that  the  free  extremities 
of  their  crowns  interlock  or  overhang  the  opposing  teeth  in  the  opposite 
jaw. 

In  the  complex  class  (Fig.  32)  the  external  form  of  the  tooth  is  pro- 
duced by  a  combination  of  cones,  some  of  which  are  simple,  others  inverted, 

but  all  uniting  at  a  common  base — the  neck 
of  the  tooth.  In  this  class  the  simple  cones 
form  the  roots  of  the  tooth,  while  the  crowns 
are  made  up  of  a  number  of  smaller  cones, 
much  modified.  Such  teeth  are  adapted  to 
crushing  and  grinding,  and  are  less  inclined 
to  interlock  during  active  service. 

The  teeth  are  divided  into  two  grand 
divisions — those  of  infancy  and  childhood, 
called  deciduous  or  temporary  teeth,  and  those 
of  the  adult  period,  known  as  permanent  teeth. 
The  latter  class  being  most  important,  will 
first  receive  consideration. 

The  Permanent  Teeth  (Fig.  33).— The 
permanent  teeth,  thirty- two  in  number,  are  divided  into  those  of  the 
superior  portion  of  the  mouth,  upper,  and  those  of  the  inferior  portion, 
lower.  In  number  they  are  equally  divided,  each  jaw  giving  support  to 
sixteen.  They  are  firmly  imbedded  in  the  alveolar  sockets  of  three  of 
the  bones  of  the  mouth,  the  upper  sixteen  being  attached  to  the  two 
superior  maxillary  or  upper  jaw-bones,  and  the  lower  sixteen  to  the 
mandible,  inferior  maxillary,  or  lower  jaw-bone.  As  above  referred  to, 
the  attachment  of  the  teeth  to  the  bones  is  by  implantation  in  sockets, 
the  alveoli  (see  description,  "Bones  of  the  Mouth").  In  this  attachment 
there  is  a  special  development  of  bone,  closely  modeled  to  the  roots  of 
the  teeth,  and  which  is  subservient  to  the  ever-varying  changes  which 
take  place  during  the  development  of  the  organs.  The  joint  thus 
formed  between  the  roots  of  the  teeth  and  the  alveoli  is  of  the  immovable 
or  synarthrodial  class,  and  is  styled  gomphosis.  Intervening  between  the 
roots  of  the  teeth  and  the  walls  of  the  alveoli  is  a  delicate  membrane — 
the  alveolodental  membrane. 

Before  continuing  the  description  of  the  teeth,  a  further  classification, 
which  refers  alike  to  the  upper  and  lower  organs,  must  be  presented. 


A    GENERAL    DESCRIPTION    OF    THE    TEETH 


77 


This  classification  is  derived  from  the  function  and  form  of  the  teeth. 
Figure  33  shows  the  thirty-two  teeth  removed  from  the  jaws  and  placed 
side  by  side  in  two  straight  lines.  In  the  center  is  a  perpendicular  line, 
which  corresponds  to  the  median  line  or  center  of  the  mouth,  the  teeth 
at  either  extremity  being  those  which  occupy  the  back  part  of  the  mouth. 
Without  confining  the  description  to  either  the  upper  or  lower  teeth,  it  will 
be  observed  that  the  first  two  teeth  upon  either  side  of  the  median  line  are 
similarly  formed,  and  all  four  are  called  incisors  (incidere,  to  cut);  the 
two  larger  incisors,  being  nearest  the  median  line  or  center,  are  called 


Lingual  Surfaces 


Labial  and  Buccal  Surfaces 


Upper 


Lower 


Lingual  Surfaces  Labial  and  Buccal  Surfaces 

Fig.  32>- — The  Permanent  Teeth. 


central  incisors;  while  the  two  smaller  being  placed  at  the  side  of  the  cen- 
trals, are  known  as  lateral  incisors  {lateralis,  the  side).  The  third  tooth 
from  the  median  line  upon  either  side  is  the  cuspid  (cuspis,  a  point),  so 
named  from  possessing  a  single  cusp  or  point.  Passing  to  the  right  or  left 
on  the  chart,  or  backward  in  the  mouth,  the  fourth  and  fifth  teeth  from 
the  median  line  are  the  bicuspids  (bi,  two;  cuspis,  a  point),  having  two 
points  or  cusps.  The  bicuspid  nearest  the  median  line  is  the  first  bicus- 
pid; that  most  distant  from  the  median  line  is  the  second  bicuspid.  The 
sixth,  seventh,  and  eighth  teeth  from  the  median  line  upon  either  side 
are  those  of  another  class,  the  molars  (mola,  a  mill-stone),  being  named 
according  to  their  function,  that  of  crushing  or  grinding  the  food.     Pro- 


78  ANATOMY 

cceding  from  before  backward,  the  molars  are  denominated  first  molar, 
second  molar,  and  third  molar.  To  sum  up,  the  names  and  number 
of  the  permanent  teeth  may  be  given  by  the  dental  formula,  as  follows: 

Incisors, i  Bicuspids,      2 

Cuspids, I  Molars,      :! 

The  Surfaces  of  the  Teeth. — The  crown  of  each  tooth  presents 
five  surfaces,  which  are  variously  named,  in  accordance  with  the  duty 
which  they  are  called  upon  to  perform  or  suggestive  of  their  location. 
The  outer  surface  of  the  incisors  and  cuspids,  or  that  contiguous  to  the 
lips  (labia),  is  called  the  labial  surface;  the  corresponding  surface  of  the 
bicuspids  and  molars,  or  that  contiguous  to  the  cheeks  (bucca?),  is  the 
buccal  surface.  That  surface  of  both  upper  and  lower  teeth  which  faces 
the  palate  or  tongue  is  characterized  as  the  lingual  surface. 

The  proximate  surfaces  of  the  teeth  are  named  with  regard  to  their 
relation  to  the  median  line,  those  surfaces  nearest  to  this  point  being 
called  mesial,  those  most  distant,  distal.  In  addition  to  these  four  sur- 
faces, which  represent  what  might  be  termed  the  sides  of  the  teeth,  a  fifth 
surface  is  present,  that  which  occludes  with  the  teeth  of  the  opposite  jaw, 
and  is  called  the  occlusal  surface. 

In  the  incisors  and  cuspids  this  surface  is  formed  by  the  converging 
of  the  labial  and  lingual  surfaces,  forming  an  edge  to  the  free  extremity  of 
the  crown,  named,  from  its  action  in  mastication,  the  incisive  or  cutting- 
edge.  In  the  bicuspids  and  molars  the  various  sides  of  the  crowns  re- 
main nearly  parallel  to  each  other  throughout  their  extent,  thus  provid- 
ing an  occlusal  surface  nearly  equal  to,  or  greater  than,  any  of  the  others, 
and  one  well  adapted  to  the  purposes  for  which  it  is  intended — that  of 
grinding  or  crushing  the  food. 

The  Roots  of  the  Teeth. — The  upper  incisors  and  cuspids  are  each 
provided  with  one  root;  the  upper  first  bicuspid  may  have  one  or  two 
roots,  most  frequently  the  latter;  while  in  the  second  bicuspid  a  single  root 
is  usually  present.  The  upper  first  and  second  molars  are  each  supported 
in  the  jaw  by  three  roots,  and  while  in  the  upper  third  molar  three  roots 
are  most  common,  the  number  is  quite  variable,  ranging  from  a  single 
cone-shaped  root  to  three,  four,  five,  or  even  six  smaller  branches  given  off 
from  a  common  base. 

In  the  lower  incisors,  cuspids,  and  bicuspids,  a  single  root  is  most 
frequently  met  with,  although  the  latter  class,  in  rare  instances,  may  be 
provided  with  two.  The  lower  first  and  second  molars  are  each  provided 
with  two  roots,  but  in  the  third  molar,  like  its  upper  fellow,  the  number 


A    GENERAL    DESCRIPTION    OF    THE    TEETH 


79 


may  be  diminished  or  increased.  In  the  upper  molars,  two  of  the  three 
roots  are  placed  above  the  buccal  half  of  the  crown,  and  are  called  buccal 
roots;  the  remaining  root  is  placed  above  the  lingual  half  of  the  crown, 
and  is  designated  as  the  lingual  root.  In  the  lower  molars,  one  of  the 
two  roots  is  placed  below  the  anterior  or  mesial  half  of  the  crown,  and  is 
named  the  mesial  root,  and  the  other  below  the  posterior  or  distal  half, 
and  is  known  as  the  distal  root.  In  those  teeth  with  a  complicated  root 
formation,  it  would  seem  to  be  a 
question  whether  they  are  pos- 
sessed of  a  single  root,  with  two 
or  more  branches,  or  separate 
and  distinct  roots  throughout. 
To  determine  this,  some  account 
must  be  taken  of  the  point  at 
which  the  bifurcation  or  trifurca- 
tion  takes  place.  If  this  separa- 
tion be  in  close  proximity  to  the 
crown,  the  tooth  should  be  con- 
sidered as  having  more  than  one 
root  (Fig.  34,  A);  but,  on  the 
other  hand,  if  the  point  of  sepa- 
ration be  some  distance  from  the 
crown,  with  a  solid  mass  of  root  substance  intervening,  the  tooth  may 
be  said  to  possess  a  single  root,  with  two  or  more  branches  (Fig.  34,  B). 
In  the  latter  instance,  that  part  of  the  tooth  between  the  point  of  sepa- 
ration and  the  crown  is  called  the  root  or  root  base;  while  the  prolonga- 
tions beyond  the  point  of  separation  are  known  as  the  branches  of 
the  root. 

The  roots  of  the  teeth  are  not  only  variable  in  number,  but  are  also 
subject  to  much  variety  in  form.  In  the  anterior  teeth  (the  incisors  and 
cuspids)  the  roots  are  inclined  to  the  form  of  the  simple  cone,  which  form, 
however,  is  frequently  more  or  less  broken  by  a  slight  curvature  near  their 
extremities,  or  by  a  slight  compression  of  their  lateral  walls.  In  the  poste- 
rior teeth  (the  bicuspids  and  molars)  the  roots,  root  bases,  or  root  branches 
are  all  inclined  to  the  conical  form,  but  do  not  approach  so  nearly  the 
perfect  cone  as  those  of  the  anterior  teeth.  These  roots  are  also  more  or 
less  crooked  and  flattened  laterally.  The  free  extremity  of  the  roots 
of  the  teeth,  forming  as  they  do  the  apex  of  these  cone-like  prolongations 
of  the  crowns,  are  known  as  the  apices  or  apical  extremities. 

The  extent  of  the  enamel  covering  to  the  crowns  of  the  teeth  is  marked 


Fig.  34. 


8o 


ANATOMY 


by  a  well-defined  line,  which  completely  encircles  the  neck  of  the  tooth, 
the  cervical  line. 

The  Dental  Arch  (Fig.  35). — The  teeth  are  arranged  in  the  jaws 
in  the  form  of  two  parabolic  curves,  the  superior  arch  describing  the 
segment  of  a  larger  circle  than  the  inferior,  as  a  result  of  which  the  upper 
teeth  slightly  overhang  the  lower.  Figure  35  represents  the  sixteen 
upper  teeth  in  position  in  the  bone,  presenting  their  occlusal  surfaces,  a 
part  of  their  lingual  surfaces  also  being  visible.  Viewed  in  this  direction, 
the  gradual  change  in  the  crowns  of  the  teeth  from  the  simple  incisors  to 


Incisors 


Cusnid              . — 

— ■ — 

HMfv 

v4 

m 

A, 

1  st  Bicuspid 

f  j*  : 

>w. 

- 

^ 

2d  Bicuspid 

r iYw*- 

y 

1st  Molai 

, 

- 

2d  Molar 

L  * 

V  1 

3d  Molar 

wT  ^k      B1'- 

Fig.  35.— The  Dental  Arch. 


the  complex  molars  may  be  observed.  An  examination  of  the  central 
incisors  will  show  how  perfectly  they  are  adapted  to  the  process  of  cutting 
or  incising  the  food,  the  cutting-edge  being  sharp  and  the  lingual  surface 
comparatively  smooth  and  unbroken.  In  the  lateral  incisors  the  cutting 
feature  predominates,  but  the  lingual  surface  is  broken  near  the  neck  of 
the  tooth  by  a  slight  depression,  surmounted  by  a  more  or  less  pronounced 
fold  of  enamel,  in  many  instances  resembling  a  small  cusp.  The  crown 
of  the  cuspid  furnishes  the  intermediate  form  between  the  simple  and 
the  complex.  This  tooth,  instead  of  being  provided  with  a  straight 
cutting-edge,  is  surmounted  at  the  center  of  its  occlusal  surface  with  a 
well-defined  point  or  cusp,  descending  from  the  summit  of  which  are  two 


A    GENERAL    DESCRIPTION    OF    THE    TEETH  8 1 

cutting-edges,  one  passing  to  the  mesial  and  one  to  the  distal.  The 
lingual  surface  of  this  tooth  presents  a  marked  contrast  to  the  correspond- 
ing surface  of  the  incisors,  being  broad  and  full,  and  frequently  provided 
with  a  prominent  ridge  of  enamel  in  the  region  of  the  neck,  showing  a 
rapid  approach  to  the  complex  form.  In  the  bicuspids  the  buccal  half 
of  the  crown  is  quite  similar  to  the  crown  of  the  cuspid,  but  in  the  lingual 
half  a  complete  revolution  has  taken  place.  The  enamel  fold — but  slightly 
apparent  in  the  incisors,  and  somewhat  increased  in  the  cuspids — has 
now  become  a  fully  developed  cusp,  resulting  in  the  production  of  an 
occlusal  surface  adapted  to  crushing  or  grinding,  instead  of  incising  or 
tearing.  In  the  molars,  the  increase  in  the  size  of  the  tooth-crown  is  ac- 
companied with  an  occlusal  surface  much  more  complex  than  any  of 
the  teeth  previously  described,  and  one  well  adapted  to  its  function — 
that  of  crushing  and  grinding  the  food. 

Arrangements  of  the  Teeth  in  the  Dental  Arch  (Fig.  35). — Beginning 
with  the  upper  teeth,  the  central  incisors  are  found  occupying  the  center 
of  the  arch,  and  are,  therefore,  slightly  in  advance  of  the  laterals.  These 
teeth  are  so  implanted  in  the  alveoli  that  their  crowns  are  not  perpendicu- 
lar, the  cutting-edge  being  slightly  more  prominent  than  the  neck  of 
the  tooth.  The  roots  are  also  somewhat  inclined  from  the  median  line, 
and  as  a  result  the  crowns  have  slight  mesial  inclination,  the  mesial  sur- 
faces approximating  each  other  at  or  near  the  cutting-edge,  with  a  slight 
space  intervening  at  the  necks.  In  certain  typal  forms — the  bilious,  for 
example — when  the  front  of  the  arch  is  flat,  the  labial  surfaces  of  these 
teeth  form  nearly  .a  direct  line  from  side  to  side;  while  in  those  types  in  which 
the  arch  is  well  rounded  anteriorly,  notably  in  the  sanguine  temperament, 
the  labial  surfaces  of  these  two  teeth  form  a  small  segment  of  the  arch, 
so  that  the  mesial  extremity  of  the  cutting-edge  of  each  tooth-crown  is 
somewhat  in  advance  of  the  distal.  The  lateral  incisors  are  similarly 
implanted  in  the  alveoli,  causing  their  cutting-edges  to  project.  The 
roots  of  the  lateral  incisors  usually  have  a  stronger  distal  inclination  than 
those  of  the  centrals,  and  the  crowns  show  a  more  marked  mesial  inclina- 
tion. The  mesial  surfaces  approximate  the  distal  surfaces  of  the  central  in- 
cisors at  or  near  the  cutting-edge.  When  the  front  of  the  arch  is  flattened, 
these  teeth  are  but  little  less  prominent  than  the  central  incisors,  but  when, 
the  arch  is  well  rounded  they  continue  the  segment  begun  by  the  centrals, 
and  are  necessarily  less  prominent.  While  the  occlusal  surfaces  of  the 
teeth  are  usually  considered  as  forming  a  perfect  plane  (see  Occlusion  of 
the  Teeth),  the  lateral  incisors  are  generally  a  trifle  shorter  than  the 
centrals.     The    cupids  may  be  considered  as  occupying  the  corners  or 


82  ANATOMY 

turning-points  of  the  arch.  They  are  more  prominently  placed  than  the 
adjoining  teeth,  this  feature  being  increased  by  the  bulging  or  general 
convexity  of  their  labial  surfaces.  The  extremity  of  the  occlusal  surface 
of  the  cuspids — i.  e.,  the  point  of  the  cusp — is  a  trifle  below  the  cutting- 
edge  of  the  laterals  and  about  on  a  line  with  that  of  the  centrals.  While 
the  apical  extremities  of  the  roots  of  the  cuspids  are  directed  away  from 
the  median  line,  the  crowns  assume  almost  a  perpendicular,  this  con- 
dition resulting  from  a  bend  in  the  tooth  at  the  neck.  Although  the 
perpendicular  position  is  most  commonly  assumed  by  the  crown,  it  is  not 
unusual  to  find  either  a  mesial  or  distal  inclination  present.  Reference 
has  been  made  to  the  cuspid  teeth  occupying  a  position  which  might  be 
termed  the  turning-points  or  corners  of  the  arch,  and  in  most  instances 
it  may  be  thus  considered;  but  in  certain  typal  forms — the  sanguine,  for 
example — the  tooth-line  is  unbroken  and  passes  over  the  cutting-edges 
of  the  incisors,  the  summit  of  the  cusps  of  the  cuspids,  and  is  continued 
backward  over  the  buccal  cusps  of  the  posterior  teeth.  The  biscuspids 
are  placed  nearly  perpendicular  in  the  arch,  but  occasionally  deviate 
from  this  by  a  slight  mesial  or  buccal  inclination.  The  length  usually 
corresponds  to  that  of  the  central  incisors,  and  their  buccal  surfaces  are 
slightly  less  prominent  than  the  corresponding  surfaces  of  the  cuspids. 
The  increase  in  the  buccolingual  diameter  of  the  crowns  of  the  bicuspids 
over  that  of  the  incisors  and  cuspids  results  in  breaking  the  lingual  line 
of  the  occlusal  surfaces.  In  the  bilious  and  kindred  types,  the  tooth- 
line  is  carried  directly  backward  from  the  cuspid  to  the  first  molar,  mak- 
ing the  buccal  face  of  the  bicuspids  equally  prominent,  but  when  the 
arch  is  well  rounded  the  second  bicuspid  is  slightly  more  prominent  than 
the  first.  The  first  and  second  molars  usually  assume  a  perpendicular 
position,  but  are  occasionally  inclined  to  the  distal  and  buccal.  The 
relative  prominence  of  the  buccal  as  well  as  the  lingual  surfaces  of  these 
teeth  is  also  controlled  by  the  form  of  the  arch.  The  occlusal  surfaces 
are  about  on  a  level  with  those  of  the  bicuspids  and  central  incisors, 
but  generally  the  lack  of  development  in  the  distal  half  of  the  crown 
of  the  second  molar  results  in  the  production  of  a  slight  upward  curve 
to  the  tooth-line  level  at  this  point  (see  Occlusion  of  the  Teeth).  On 
account  of  the  limited  accommodations  afforded  it,  the  position  of  the 
upper  third  molar  is  quite  variable.  It  may  be  either  to  the  buccal 
or  to  the  lingual  of  the  tooth-line,  and  is  usually  strongly  inclined  to 
the  distal.  In  those  cases  in  which  there  is  a  decided  dip  to  the  arch 
(see  Occlusion  of  the  Teeth),  this  tooth  is  relatively  shorter  than  those  an- 


/ 


A    GENERAL    DESCRIPTION    OF    THE    TEETH 


83 


terior  to  it,  but  when  the  tooth-line  level  is  a  perfect  plane,  the  length  of 
this  tooth  corresponds  to  the  other  molars  and  bicuspids. 


Fig.  36. — The  Tooth-line  in  the  Lower  Jaw. 

The  lower  incisors  are  placed  more  nearly  in  a  perpendicular  position 
than  the  upper,  and  a  reverse  condition  exists,  in  the  lateral  incisors 


Fig.  37. — The  Tooth-line  in  the  Lower  Jaw. 

being  a  trifle  larger  than  the  centrals.     The  lower  cuspids  are  probably 
more  constant  in  their  position  than  any  class  of  teeth  in  the  mouth,  in 


84 


ANATOMY 


nearly  all  instances  assuming  a  direct  perpendicular.  Like  the  upper  cus- 
pids, they  may  be  said  to  establish  the  corners  or  turning-points  of  the 
lower  arch,  and  are  somewhat  more  prominent  in  the  tooth-line  than 


Fig.  38. — Sanguine. 

neighboring  teeth.  All  of  the  six  anterior  lower  teeth  may  be  slightly 
inclined  to  the  mesial.  The  lower  biscupids  and  molars,  instead  of 
having  the  buccal  inclination  possessed  by  the  corresponding  upper  teeth, 


Fig.  39.— Bilious. 

are  inclined  to  the  lingual.  The  first  molar  seldom  deviates  either  to  the 
mesial  or  the  distal,  the  second  molar  is  generally  inclined  to  the  mesial, 
while  the  third  molar  is  strongly  inclined  to  the  mesial.     In  the  lower 


A   GENERAL   DESCRIPTION    OF    THE    TEETH 


85 


arch  the  curve  formed  by  the  incisors  and  cuspids  is  the  segment  of  a 
smaller  circle  than  the  corresponding  curve  in  the  upper  arch.  This 
curve  may  be  continued  over  the  buccal  cusps  of  the  bicuspids  and  molars, 


Fig.  40. — Nervous. 


or  it  may  be  broken  at  the  cuspid  and  continued  backward  in  a  direct 
line  (Fig.  36).  The  teeth  in  the  lower  arch  are  placed  directly  over  the 
body  of  the  bone  as  far  back  as  the  second  bicuspids,  while  the  molars 


Fig.  41. — Lymphatic. 

frequently  overhang  the  body  of  the  bone  by  an  extension  of  the  alveoli 
inward  (Fig.  37). 

The  curve  described  by  the  dental  arch  is  quite  variable,  and  this 


86 


ANATOMY 


variation  is  generally  referred  to  in  connection  with  the  temperament. 
Thus,  in  the  sanguine  temperament  (Fig.  38),  the  arch  is  well  rounded 
anteriorly,  the  circle  being  continued  backward  to  the  region  of  the  molars, 
where  the  line  is  broken  by  slightly  inclining  to  the  lingual.  In  this 
arch  the  distance  in  a  straight  line  from  the  center  of  the  second  molar, 
on  one  side,  to  the  center  of  the  corresponding  tooth  on  the  other,  is  about 
equal  to  the  distance  from  either  of  these  points  to  the  median  line  be- 
tween the  central  incisors,  forming  a  right-angle  triangle.  In  the  bilious 
temperament  (Fig.  39)  the  arch  presents  a  broad  front  from  cuspid  to 


Fig.  42. — Section  of  the  Superior  Maxilla  Showing  Tnterproximate  Spaces. 


cuspid,  with  but  little  curve;  at  these  points  it  turns  abruptly  backward, 
being  continued  almost  in  a  direct  line  to  its  extremity.  In  this  arch 
the  side  of  the  triangle  (represented  by  the  line  from  molar  to  molar) 
is  much  reduced  in  length.  In  the  nervous  temperament  (Fig.  40) 
the  arch  is  Gothic  in  form,  the  segment  formed  by  the  anterior  teeth 
being  that  of  a  much  smaller  circle  than  either  of  the  types  previously 
referred  to.  The  distance  from  molar  to  molar  is  much  less  than  the 
distance  from  molar  to  median  line.  In  the  lymphatic  temperament 
(Fig.  41)  the  arch  is  well  rounded  and  broad,  the  segment  being  that 
of  a  much  larger  circle  than  any  of  the  above,  the  side  of  the  triangle 
formed  by  the  line  from  molar  to  molar  being  of  the  greatest  length. 
Interproximate  Spaces. — In  the  mesiodistal  direction  the  crowns 
of  the  teeth,  as  a  class,  are  broader  at  their  occlusal  surfaces  or  cutting- 
edges  than  at  their  necks  (Fig.  42).  This  bell-shaped  form  of  the  tooth- 
crowns  cause  their  proximate  surfaces  to  touch  at  a  point  representing 


A    GENERAL    DESCRIPTION    OF    THE    TEETH  87 

their  greatest  mesiodistal  diameter,  which  is  usually  near  the  cutting-edge 
or  occlusal  surface.  Between  this  point  of  contact  and  the  cervical  line 
there  exists  a  V-shaped  space,  called  the  interproximate  space.  These 
spaces  are  largest  in  that  class  of  teeth  found  in  the  nervous  and  bilious 
types,  where  the  necks  of  the  teeth  are  much  constricted,  and  the  bell- 
shaped  crown  strongly  outlined.  In  teeth  of  this  class  the  point  of  contact 
is  slight,  and  the  interproximate  spaces  are  only  partially  occupied  by 
the  gum  tissue,  leaving  a  free  passage  between  the  point  of  contact  and 
the  gingival  margins.  In  the  sanguine  and  lymphatic  temperaments  the 
proximate  surfaces  of  the  teeth  are  nearer  parallel  with  one  another,  thus 
making  the  point  of  contact  cover  a  greater  extent  of  surface,  and  reducing 
the  size  of  the  interproximate  spaces. 


CHAPTER  VI. 


Occlusion  of  the  Teeth. 


As  stated  elsewhere,  the  teeth  are  arranged  in  the  mouth  in  the  form 
of  two  parabolic  curves,  one  of  which  occupies  the  upper  half  and  the 
other  the  lower  half  of  the  cavity.  To  properly  perform  their  function  it 
is  necessary  for  the  upper  and  lower  teeth  to  come  into  contact,  which 


Fig.  43. — The  Teeth  in  Occlusion. 

they  are  enabled  to  do  by  the  movement  of  the  lower  jaw,  and  it  is  the 
relation  existing  between  the  two  when  thus  brought  together  that  con- 
stitutes the  occlusion  of  the  teeth.  During  mastication  the  teeth  do  not 
only  occlude,  and  remain  stationary  at  a  given  point  until  the  lower  jaw 
is  again  depressed,  but,  through  the  combined  movements  of  the  man- 
dible, the  lower  teeth  are  made  to  move  from  side  to  side,  thus  grinding 
or  crushing  any  substance  placed  between  the  occlusal  surfaces  of  the 
bicuspids  and  molars.  This  gliding  antagonism  of  the  teeth  is  commonly 
termed  the  articulation,  and  it  is  important  that  a  distinction  be  made  be- 


OCCLUSION    OF    THE    TEETH  89 

tween  the  terms  "occlusion"  and  "articulation,"  the  former  referring  to 
the  relations  existing  between  the  upper  and  lower  teeth  when  brought 
together  normally  and  held  firmly  in  that  position,  while  the  latter  relates 
to  the  various  gliding  movements  of  the  lower  teeth  after  being  brought 
into  occlusion  with  the  upper.  In  the  majority  of  instances  the  segment 
described  by  the  upper  arch  is  somewhat  larger  than  that  formed  by  the 
lower,  and  the  upper  teeth  project  over  and  are  partly  outside  of  those  in 
the  lower  arch.  Figure  43  presents  a  labial  and  buccal  view  of  the  teeth  in 
position  in  the  alveoli,  and  also  in  occlusion.  It  will  be  observed  that  the 
upper  teeth  are  not  directly  antagonistic  to  those  of  the  same  name  in  the 
lower  arch.  There  are  two  reasons  for  the  presence  of  this  condition: 
First,  the  mesiodistal  diameter  of  the  upper  central  incisors  is  much 
greater  than  that  of  the  corresponding  lower  teeth;  second,  the  larger 
circle  present  in  the  upper  arch.  This  arrangement  provides  that 
each  tooth,  instead  of  being  antagonized  by  a  single  tooth  of  the  opposite 
jaw,  is  met  in  occlusion  by  a  portion  of  two  teeth.  The  upper  central 
incisor  is  met  in  occlusion  by  the  entire  cutting-edge  of  the  lower  central 
incisor  and  the  mesial  third  of  the  cutting-edge  of  the  lower  lateral  in- 
cisor. The  upper  lateral  incisor  is  met  in  occlusion  by  the  distal  two-thirds 
of  the  cutting-edge  of  the  lower  lateral  incisor  and  by  the  mesial  cut- 
ting-edge of  the  lower  cuspid.  The  upper  cuspid  is  met  in  occlusion  by 
the  distal  cutting-edge  of  the  lower  cuspid  and  by  the  mesial  two-thirds 
of  the  buccal  cusp  of  the  lower  first  bicuspid.  The  upper  first  biscuspid 
is  met  in  occlusion  by  the  remaining  or  distal  third  of  the  lower  first  bicus- 
pid and  by  the  mesial  two-thirds  of  the  buccal  cusp  of  the  lower  second 
bicuspid.  The  upper  second  bicuspid  is  met  in  occlusion  by  the  re- 
maining or  distal  third  of  the  buccal  cusp  of  the  lower  second  bicuspid, 
and  by  the  mesial  incline  of  the  mesiobuccal  cusp  of  the  lower  first 
molar.  The  upper  first  molar  is  met  in  occlusion  by  the  distal  incline 
of  the  mesiobuccal  cusp  of  the  lower  first  molar,  by  the  entire  distal 
cusp  of  the  same  tooth,  and  by  the  mesial  incline  of  the  mesiobuccal  cusp 
of  the  lower  second  molar.  The  upper  second  molar  is  met  in  occlusion 
by  the  distal  incline  of  the  mesiobuccal  cusp  of  the  lower  second  molar, 
by  the  entire  distobuccal  cusp  of  the  same  tooth,  and  by  the  mesial  incline 
of  the  mesiobuccal  cusp  of  the  lower  third  molar.  The  upper  third 
molar  is  met  in  occlusion  by  the  distal  incline  of  the  mesiobuccal  cusp  of 
the  lower  third  molar  and  by  the  entire  distobuccal  cusp  of  the  same  tooth, 
thus  being  the  only  tooth  in  the  upper  arch  with  but  a  single  opponent. 
Likewise  each  lower  tooth  is  met  in  occlusion  by  two  in  the  superior  arch, 
with  the  single  exception  of  the  central  incisor,  which  occludes  with  the 


go 


ANATOMY 


upper  central  alone.  There  are  many  variations  from  this  so-called 
typical  occlusion,  as  above  described,  and  any  slight  difference  one  way 
or  another  should  not  be  considered  abnormal.  In  certain  types  the 
segmental  form  of  the  upper  arch  is  but  little  greater  than  that  of  the 
lower,  and  the  cutting-edges  of  the  upper  incissors  occlude  almost  directly 
upon  the  cutting-edges  of  the  lower  incisors.  As  a  result,  all  of  the  upper 
teeth  are  forced  to  the  distal,  and  the  relationship  between  the  upper  and 
lower  organs  is  much  altered.  When  the  upper  teeth  overhang  the  lower, 
the  lingual  cusps  of  the  upper  bicuspids  and  molars  penetrate  the  fossae 


Fig.  44. 


-The  Mandible  at  the  Adult  Period,  showing  the  Equilateral  Triangle 
described  by  the  Dental  Arch. 


or  sulci  of  the  corresponding  lower  teeth,  when  in  occlusion,  and  the 
buccal  cusps  of  the  lower  bicuspids  and  molars  rest  in  the  fossae  of  their 
upper  opponents. 

To  assist  in  the  study  of  the  occlusion  of  the  teeth,  some  reference 
must  be  made  to  the  tooth-line  lever,  or  plane  of  occlusion.  For  this 
purpose  the  lines  forming  the  facial  angle  are  of  value.  These  lines  are 
as  follows:  A  fixed  line  representing  the  base  of  the  angle  may  be  drawn 
from  the  center  of  the  glenoid  fossa,  passing  forward  through  the  anterior 
nasal  spine  or  base  of  the  nose  (a,  Fig.  45),  the  angle  being  completed  by  a 
perpendicular  line  resting  upon  the  labial  surface  of  the  upper  incisors, 
passing  upward  and  touching  the  most  prominent  part  of  the  forehead  (b, 
Fig.  45).  The  tooth-line  level  is  approximately  horizontal  to  this  basal 
line,  but  instead  of  a  perfect  plane  we  usually  find  the  upper  arch  dipping 
downward,  while  the  lower  arch  will  be  provided  with  a  corresponding 


OCCLUSION    OF    THE    TEETH 


91 


depression.  This  dip  to  the  arch  which  is  known  as  the  "compensating 
curve,"  or  the  "curve  of  Spee,"  is  greatest  in  the  region  of  the  bicuspisd, 
and  the  extent  to  which  it  may  exist  varies  with  the  type  of  tooth  and  the 
consequent  nature  of  the  occlusion. 


Fig.  45. — Lines  showing  Facial  Angle,  Caucasian  or  White  Race. 

Thus  far  no  reference  has  been  made  to  what  is  commonly  termed 
the  overbite,  and  the  cusp  forms  in  the  teeth.     As  these  two  factors  exert 


Fig.  46. — Lines  showing  Facial  Angle,  Negro  or  Mixed  Races. 

a  dominating  influence  over  the  character  of  the  occlusion,  the  effects  which 
they  produce  will  be  briefly  described.     The  overbite  is  so  named,  from  the 


92  ANATOMY 

fact  that  the  upper  teeth  project  beyond,  or  overhang  and  partly  cover  the 
labial  and  buccal  surfaces  of  the  lower  teeth.  This  may  be  a  pronounced 
feature  in  the  occlusion,  or  it  may  exist  to  a  very  slight  degree.  Al- 
though the  overbite  is  usually  referred  to  as  existing  in  the  incisive 
region  alone,  it  is  not  confined  to  these  teeth,  but  is  also  present  in  the 
bicuspids  and  molars  by  the  buccal  cusps  of  the  upper  teeth  overhanging 


Fig.  47. — Lymphatic. 

those  of  the  lower.  The  extent  of  the  overbite  is  gradually  diminished 
from  before  backward,  the  central  incisors  presenting  the  greatest  amount 
of  overhanging  surface,  which  condition  is  slightly  decreased  in  the 
laterals,  and  a  corresponding  reduction  is  continued  until  the  third 
molars  are  reached,  at  which  point  the  overbite  is  scarcely  observed. 
Where  the  overbite  is  extensi/e,  as  shown  in  figure  43,  the  upper  incisors 


Fig.  48. — Sanguine. 

overhanging  and  hiding  from  view  about  one-third  of  the  labial  surfaces 
of  the  lower  incisors,  the  cusps  of  the  bicuspids  and  molars  will  be  corre- 
spondingly long  and  penetrating,  the  buccal  cusps  of  the  upper  teeth  ex- 
tending well  down  over  the  buccal  cusps  of  the  corresponding  lower 
teeth.  In  an  occlusion  of  this  class,  which  is  usually  found  in  the  nerv- 
ous  and  bilious   types,   the  dip  to  the  arch  will  become  a  prominent 


OCCLUSION    OF    THE    TEETH. 


93 


feature,  the  occlusion  will  be  firm  and  well  locked,  and  the  lateral  articular 
movements  will  be  slight  during  mastication.  In  the  lymphatic  and  san- 
guine temperaments  the  occlusion  is  loose  and  wandering,  greater  freedom 
of  movement  being  permitted  by  the  short  overbite  and  the  corresponding 
lack  of  cusp-formation.     Figure  47  represents  such  an  occlusion;  the 


Fig.  49. — Bilious. 

superior  arch  is  but  little  greater  in  its  segmental  outline  than  the  inferior. 
The  cutting-edges  of  the  upper  incisors  are  somewhat  more  prominent 
than  those  of  the  lower,  but  the  former  do  not  overlap  the  labial  surfaces 
of  the  latter.  In  an  occlusion  of  this  character  the  dip  to  the  arch  is  not 
so  pronounced,  and  the  articular  movements  are  much  more  extensive. 


CHAPTER  VII. 

The  Blood-  and  Nerve-Supply  to  the  Teeth. 

THE  BLOOD-SUPPLY  TO  THE  TEETH. 

Briefly  stated,  the  course  of  the  blood  from  the  heart  to  the  teeth  is 
as  follows:  From  the  heart  to  the  aorta,  to  the  common  carotid  artery, 
to  the  external  carotid  artery,  to  the  internal  maxillary  artery,  from  the 
various  branches  of  which  the  teeth  are  supplied. 

The  Internal  Maxillary  Artery  (Fig.  51). — This  artery,  otherwise 
known  as  the  deep  facial,  is  the  larger  of  the  two  terminal  branches  of 
the  external  carotid.  In  addition  to  supplying  the  teeth,  it  is  distributed 
to  the  roof  and  floor  of  the  mouth,  to  the  maxillary  sinus,  and  to  other 
parts  of  the  face  and  head.  It  has  its  origin  from  the  external  carotid 
artery  opposite  the  condyle  of  the  mandible  within  the  substance  of  the 
parotid  gland,  passes  forward  between  the  condyle  of  the  jaw  and  the 
sphenomaxillary  ligament,  from  which  point  it  passes  obliquely  upward 
and  forward  between  the  external  and  internal  pterygoid  muscles  until 
it  reaches  the  sphenomaxillary  fossa,  where  its  terminal  branches  are 
given  off.  It  is  divided  into  three  portions — the  first  or  maxillary, 
the  second  or  pterygoid,  and  the  third  or  sphenomaxillary.  The  teeth 
are  supplied  from  branches  of  the  first  and  third  divisions,  the  upper 
teeth  receiving  their  blood-supply  from  the  alveolar  or  superior  maxillary 
and  the  infra-orbital  branches  of  the  third  division,  while  the  lower 
teeth  are  supplied  by  the  inferior  dental  or  mandibular  branch  of  the  first 
division. 

The  alveolar  or  superior  maxillary  branch  arises,  in  common 
with  the  infra-orbital  branch,  from  the  internal  maxillary  as  it  passes  into 
the  sphenomaxillary  fossa.  It  passes  downward,  in  a  tortuous  manner, 
in  a  groove  provided  for  it  in  the  back  of  the  maxilla.  In  its  downward 
course  it  gives'  off  the  following  branches:  The  antral,  to  supply  the  an- 
trum; the  dental  (known  as  the  posterior  dental  arteries),  which  pass  into 
the  substance  of  the  bone  through  the  posterior  dental  canals  to  supply 
the  molar  and  bicuspid  teeth;  the  alveolar  or  gingival,  to  supply  the  gums; 
and  the  buccal,  to  the  lateral  walls  of  the  mouth.  The  anterior  upper 
teeth  are  supplied  through  the  infra-orbital  branch  of  the  internal  maxillary. 

94 


Fig.  50. — Dissection  Showing  Blood-supply  to  the  Teeth. 


9S 


96 


ANATOMY 


This  branch  arises  from  the  internal  maxillary  artery,  generally,  in 
common  with  the  posterior  dental.  It  passes  forward  in  company  with 
the  maxillary  division  of  the  fifth  nerve — first  along  the  groove  and 
then  in  the  canal  on  the  orbital  plate  of  the  maxilla,  and  finally  makes 
its  exit  upon  the  face  through  the  infra-orbital  foramen.  Besides  giving 
off  branches  to  the  orbital  and  nasal  cavities,  it  supplies  the  incisor  and 
cuspid  teeth  through  its  anterior  dental  branch,  which  passes  downward 
through  a  groove  in  the  anterior  wall  of  the  maxilla. 


Infraorbital  artery  and  nerve 


Spheno-pqlaline  branch 

Posterior  or  descending  palatine  branch 
Naso-palatine  branch 

Vidian  branch 

A  nterior  deep  temporal  artery 

External  pterygoid  branch 


Orbital  branch 


Nasal  branch 

A nterior 

dental  branch 

Labial  branch 

Posterior  dental 

branch 


Posterior  deep/  temporal  artery 

Small  meningeal 
artery 

Middle  meningeal 
artery 


Temporal  artery 
Tympanic  branch 

Deep  auricular 

branch 
A  URICULO-  TEM- 
PORAL NERVE 

—  Masseteric  branch 


Incisive  branch 
Menial  branch 


Submental  branch 


External  carotid 
artery 


Internal  lateral  or 
Bpheno-mandibu- 
lar  ligament 

Mandibular  or 
inferior  dental 
artery  and  nerve 


Fig.  51. 


Buccal  branch  with 
portion  of  buccal  nerve 

Mylo-hyoidean  branch 
-Scheme  of  Internal  Maxillary  Artery.     (Morris.) 


Internal  pterygoid  branch 


The  Inferior  Dental  Artery  (Fig.  51). — The  lower  teeth  receive 
their  blood-supply  through  the  inferior  dental  or  mandibular  artery. 
This  artery  arises  from  the  under  part  of  the  internal  maxillary  as  it 
passes  downward  and  forward  between  the  sphenomaxillary  ligament 
and  the  neck  of  the  jaw  and  enters  the  inferior  dental  canal  through  the 
inferior  dental  foramen.  It  passes  forward  in  the  canal  accompanied 
by  the  inferior  dental  nerve,  and  in  so  doing  sends  off  twigs  to  supply 


Fig.  52. — Dissection  Showing  Nerve-supply  to  the  Teeth. 
97 


98  ANATOMY 

the  molar  and  bicuspid  teeth.  When  the  mental  foramen  is  reached, 
it  divides  into  two  branches,  the  incisive  branch  and  the  mental  branch. 

The  incisive  branch  continues  it  course  within  the  cancellated  struc- 
ture of  the  bone,  sending  off  minute  branches  which  supply  the  anterior 
teeth,  the  terminal  branches  anastomosing  with  the  artery  of  the  opposite 
side. 

The  mental  branch  passes  out  through  the  mental  foramen  accom- 
panied by  the  mental  branches  of  the  inferior  dental  nerve,  and  supplies 
the  tissues  of  the  chin  and  lower  lip. 

The  Veins. — The  blood,  in  returning  from  the  teeth  to  the  heart, 
is  first  taken  up  by  the  posterior  dental  and  inferior  dental  veins,  which 
in  their  course  follow  closely  that  of  their  corresponding  arteries.  These 
veins,  in  conjunction  with  others  which  accompany  branches  of  the  inter- 
nal maxillary  artery,  form  the  pterygoid  plexus.  At  the  posterior  con- 
fluence of  this  plexus  the  returning  blood  empties  into  the  internal  maxil- 
lary vein.  Accompanied  by  the  internal  maxillary  artery  it  passes  back- 
ward and  outward,  enters  the  parotid  gland,  and  finally  empties  into  the 
temporomaxillary  vein  midway  between  the  zygoma  and  the  angle  of  the 
jaw.  After  leaving  the  substance  of  the  parotid  gland,  the  temporo- 
maxillary vein  passes  downward  until  near  the  angle  of  the  jaw,  where 
it  divides  into  two  branches,  one  of  which  passes  downward  and  slightly 
forward,  uniting  with  the  facial  to  form  the  common  facial  vein,  and 
the  other,  after  passing  downward  and  backward,  empties  into  the 
external  jugular  vein.  The  external  jugular  vein  returns  the  principal 
portion  of  the  blood  from  the  teeth,  and  from  its  point  of  beginning  it 
passes  almost  perpendicularly  downward  and  empties  into  the  sub- 
clavian vein,  which,  by  joining  with  the  internal  jugular  vein,  forms  the 
innominate  vein,  which,  in  turn,  empties  into  the  superior  vena  cava, 
thus  communicating  with  the  heart. 

THE  NERVE-SUPPLY  TO  THE  TEETH. 

The  nerves  supplying  the  teeth  are  derived  from  branches  of  the 
fifth  cranial  nerve,  otherwise  known  as  the  trifacial  or  trigeminal  nerve. 
The  fifth  nerve  is  the  largest  of  the  cranial  nerves,  and  consists  of  two  parts, 
a  large  root  (sensory)  and  a  small  root  (motor).  The  larger  portion  passes 
into  a  ganglion  (the  Gasserian  ganglion),  frequently  compared  to  the 
ganglion  on  the  posterior  root  of  the  spinal  nerve.  It  arises,  or  makes  its 
appearance,  at  the  surface  of  the  brain,  on  the  anterior  part  of  the  side 
of  the  pons  Varolii.     The  sensory,  root,  which,  through  its  branches, 


THE  BLOOD-SUPPLY  TO  THE  TEETH 


99 


supplies  the  teeth,  is  composed  of  from  80  to  100  filaments,  each  inclosed 
in  a  neurilemma,  the  entire  bundle  being  bound  together  in  a  single  nerve. 

The  fifth  nerve  is  divided  into  three  divisions:  First,  or  ophthalmic; 
second,  or  superior  maxillary;  and  third,  or  inferior  maxillary  (mandibu- 
lar) .  The  branches  which  supply  the  teeth  are  included  in  the  second  and 
third  divisions,  the  upper  teeth  being  supplied  by  branches  from  the 
superior  maxillary  nerve,  and  the  lower  teeth  by  branches  from  the  in- 
ferior maxillary  nerve. 

The  Second  Division,  or  Superior  Maxillary  Nerve  (Fig.  53). — 
This  nerve,  composed  entirely  of  sensory  fibers,  is  intermediate  in  size 


ANTERIOR  DENTAL        MAXILLARY  NERVE        ORBITAL  BRANCH 


MA  XILLAR  Y  NER  VE 


MECKEL'S  GANGLION 


POSTERIOR  DENTAL 


LOOP  FORMED  BY  MIDDLE  AND  ANTERIOR  DENTAL  NERVES 
Fig.  $$. — The  Maxillary  Nerve,  seen  from  Without.     {Morris,  after  Beawris.) 


between  the  inferior  maxillary  and  the  ophthalmic  divisions.  It  passes 
forward  from  the  Gasserian  ganglion  and  leaves  the  cranium  through  the 
foramen  rotundum.  It  traverses  the  upper  part  of  the  sphenomaxillary 
fossa,  and  passes  into  the  orbit  through  the  sphenomaxillary  fissure;  then 
passes  forward  along  the  infra-orbital  groove,  and  enters  the  infra- 
orbital canal,  where  it  receives  the  name  of  the  infra-orbital  nerve.  Pass- 
ing through  this  canal,  it  emerges  upon  the  face  through  the  infra-orbital 
foramen.  The  superior  maxillary  nerve,  beside  supplying  the  teeth, 
sends  off  branches  to  the  dura  mater,  to  the  orbit,  and  terminal  branches 
in  three  groups — labial,  nasal,  and  palpebral.     The  branches  given  off 


TOO 


ANATOMY 


to  the  teeth  are  the  posterior  superior  dental,  the  middle  superior  dental, 
and  the  anterior  superior  dental. 

The  posterior  superior  dental  arises  from  the  second  division  of  the 
fifth  nerve,  by  one  or  two  roots,  just  before  it  passes  into  the  infra-orbital 
canal.  It  is  divided  into  a  superior  and  an  inferior  set;  the  former  passes 
forward  and  terminates  in  the  canine  fossa,  while  the  latter,  usually  the 
larger,  enters  the  posterior  dental  canals,  and,  following  the  line  of  the 


Fig.  54. — Dissection  showing  Mandibular  (Third)  Division  of  Fifth  Nerve,  a,  Temporal 
Bone;  b,  Mental  Branches  emerging  through  Mental  Foramen;  c,  Lingual  Nerve;  d,  Man- 
dibular or  Inferior  Dental  Nerve. 


alveolar  process  through  minute  canals  in  the  bone,  sends  off  twigs  to  the 
molar  teeth,  ending  in  a  plexiform  manner  by  communicating  with  the 
middle  superior  dental  nerve.  This  nerve  is  also  distributed  to  the  gums 
and  adjacent  buccal  mucous  membrane. 

Middle  Superior  Dental  Nerve. — The  infra-orbital  nerve,  soon  after 
entering  its  canal,  gives  off  this  branch,  which  passes  outward,  downward, 
and  forward  over  the  outer  wall  of  the  maxillary  sinus,  and,  after  forming 


THE    NERVE-SUPPLY    TO    THE    TEETH  IOI 

plexuses  with  the  posterior  dental  branches,  gives  off  filaments  to  supply 
the  bicuspid  teeth. 

The  anterior  superior  dental  nerve,  which  is  the  largest  of  the  dental 
set,  is  given  off  from  the  infra-orbital  nerve,  enters  a  canal  close  to  the 
infra-orbital  foramen,  passes  over  the  anterior  wall  of  the  maxillary 
sinus,  and,  after  communicating  with  the  middle  and  posterior  dental 
nerves,  divides  into  ascending  and  descending  branches,  the  latter  being 
distributed  to  the  incisor  and  cuspid  teeth. 

The  Third  Division,  or  Inferior  Maxillary  Nerve  (Fig.  54). — 
This  is  the  largest  of  the  three  divisions  of  the  fifth  nerve,  and  is  both 
motor  and  sensory  in  its  function.  Besides  being  distributed  to  the 
lower  teeth,  it  sends  filaments  to  the  lower  portion  of  the  face,  the  muscles 
of  mastication,  the  tongue,  and  mandible.  It  arises  from  the  Gasserian 
ganglion,  passes  downward,  and  emerges  from  the  skull  through  the  fora- 
men ovale,  after  which  it  divides  into  a  small  anterior  (motor)  branch 
and  a  large  posterior  (sensory)  branch. 

The  Inferior  Dental  Nerve. — This  is  the  largest  branch  of  the  inferior 
maxillary  nerve.  From  its  point  of  origin  it  passes  downward  internally  to 
the  external  pterygoid  muscle,  and,  upon  reaching  a  point  between  the 
ramus  of  the  mandible  and  the  sphenomandibular  ligament,  it  enters 
the  inferior  dental  canal  through  the  posterior  or  inferior  dental  foramen. 
Before  entering  the  foramen,  two  branches  are  given  off,  a  lingual  and  a 
mylohyoid  branch.  The  nerve  is  accompanied  through  the  inferior 
dental  canal  by  the  inferior  dental  artery,  and,  when  the  mental  foramen 
is  reached,  it  terminates  by  dividing  into  an  incisive  and  a  mental  branch. 
Between  the  posterior  dental  foramen  and  the  mental  foramen  the  nerve 
gives  off  a  series  of  twigs  to  the  bicuspid  and  molar  teeth,  and  these,  by 
communicating  with  one  another  within  the  substance  of  the  bone,  form 
a  fine  plexus. 

The  incisive  branch  follows  the  incisive  arteries  through  the  sub- 
stance of  that  part  of  the  bone  between  the  mental  foramen  and  the 
symphysis,  and  supplies  the  incisor  and  bicuspid  teeth,  while  the  mental 
branch  passes  forward  to  supply  the  chin  and  lower  lip. 


CHAPTER  VIII. 

A  Description  of  the  Upper  Teeth  in  Detail. — Calcification,  Erup- 
tion, and  Average  Measurements. — Their  Surfaces,  Ridges, 
Fossae,  Grooves,  Sulci,  Etc. 

UPPER  CENTRAL  INCISOR. 


ist  3d  6th  7th  8th  9th  10th  nth 

year  year  year  year  year  year  year  year 

Fig.  55. 
Calcification  Begins,  from  Three  Centers,  First  Year  after  Birth. 
Calcification  Completed,  Tenth  to  Eleventh  Year. 
Erupted,  Seventh  to  Eighth  Year. 
Average  Length  of  Crown,  .39. 

Average  Length  of  Root,  .49. 

Average  Length  over  All,  .88. 

During  the  first  year  after  birth  this  tooth  begins  to  calcify,  this 
process  taking  place  along  the  future  cutting-edge  of  the  tooth  in 
three  distinct  lobes  or  plates,  which  afterward  unite  and  form  three 
eminences  or  tubercles,  the  lines  of  this  union  being  indicated  upon  the 
completed  crown  by  two  more  or  less  defined  grooves — developmental 
grooves.  By  the  end  of  the  third  year  the  deposit  of  lime-salts  has  carried 
the  process  of  calcification  to  a  point  about  midway  between  the  cutting- 
edge  and  the  cervical  line.  By  a  continuation  of  this  formative  action 
the  calcification  of  the  crown  is  completed  between  the  fifth  and  sixth 
year.  At  the  beginning  of  the  seventh  year  calcification  has  progressed 
to  such  an  extent  that  the  neck  of  the  tooth  and  base  of  the  root  are  fully 
outlined.  Between  the  seventh  and  eighth  year  the  cutting-edge  of  the 
tooth  begins  to  make  its  appearance  through  the  gum  at  a  point  either  to 
the  right  or  left  of  the  median  line,  and,  by  a  gradual  absorption  of  the 


UPPER    CENTRAL    INCISOR 


IO3 


gum  tissue,  eruption  takes  place.  During  the  following  year  about  one- 
eighth  of  an  inch  has  been  added  to  the  length  of  the  root.  At  the  end  of 
the  eighth  year  the  root  has  become  calcified  to  about  one-half  of  its 
completed  length.  During  the  ninth  year,  owing  to  a  reduction  in  the 
diameter  of  the  root,  the  extent  of  growth  has  almost  doubled  that  of 
the. previous  year,  and  a  decided  narrowing  of  the  free  root  margins  is 
to  be  observed.  At  the  eleventh  year  calcification  is  completed  in  the 
outer  root  walls  (Fig.  55). 

The  Crown  of  the  Upper  Central  Incisor  presents  for  examination 
four  surfaces — libial,  lingual,  mesial,  and.  distal;  two  angles — a  mesial 
and  a  distal;  and  a  cutting-edge.  The 
general  form  of  the  crown  is  that  of  a 
double  inclined  plane,  or  wedge-shape, 
the  cutting-edge  representing  the  junc- 
tion of  the  two  sides  of  the  incline,  one 
of  which  looks  anteriorly  (labial)  and 
the  other  posteriorly  (lingual).  The 
labial  side  of  the  incline  is  convex, 
while  the  lingual  is  concave  from  the 
cutting-edge  toward  the  root:  but, 
upon  reaching  its  upper  or  cervical 
third,  it  presents  a  slight  general  con- 
vexity. The  base  of  the  wedge  is 
directed  upward  and  partakes  of  the 
contour  of  the  neck  of  the  tooth. 

The  Labial  Surface  of  the  Crown 
(Fig.  56). — In  general  outline  his  surface  resembles  an  imperfect  quad- 
rilateral. The  margins  of  the  surface  are  the  mesial,  the  distal,  the 
cervical,  and  the  incisive.  The  mesial  margin  begins  at  the  lower  border 
or  cutting-edge  and  passes  upward,  usually  with  a  slight  distal  incli- 
nation, gradually  uniting  with  the  cervical  margin.  The  distal  margin 
begins  at  the  cutting-edge  and  passes  upward  with  a  slight  mesial 
inclination,  also  joining  the  cervical  margin.  Both  of  these  margins 
possess  more  or  less  general  convexity,  and,  at  their  junction  with  the 
cutting-edge,  form  the  mesial  and  distal  angles  of  the  crown.  The 
cervical  margin  is  rounded  and  gradually  passes  into  the  two  lateral 
margins  just  described.  The  incisive  margin  is  marked  by  the  cutting- 
edge,  and  extends  from  the  mesial  angle  on  one  side  to  the  distal 
angle  on  the  other.  These  four  margins,  which  assist  in  giving  to  the 
tooth  its  typal  form,   are  quite  variable.     This  difference  particularly 


Mesia 
Angle 


Distal. 
Angle 


Cutting    Labial 
Edge    Grooves 

Fig.  56. — Upper  Central  Incisor, 
Labial  Surface. 


104 


ANATOMY 


Cervical 

Ridge 

Distal 

Marginal 

Ridge 

Distal 
Angle 


marked  on  the  mesial  and  distal  margins, where,  in  some  cases,  there 
is  a  decided  convergence  in  the  direction  of  the  root,  forming  what  is 
commonly  termed  the  bell-shaped  crown,  while  in  others  the  same 
margins  will  be  nearly  parallel  with  each  other,  making  the  width  of  the 
crown  almost  as  great  at  the  cervical  margin  as  at  the  cutting-edge.  The 
mesial  angle  is  usually  pointed  and  square,  while  the  distal  is  much 
rounded.  This  surface  of  the  crown  is  slightly  convex  from  above  down- 
ward, as  well  as  from  side  to  side,  and  in  the  majority  of  instances  is  of 
greater  vertical  than  transverse  extent.     Beginning  at  the  incisive  margin 

are  two  slight  longitudinal  depressions 
or  grooves — the  labial  grooves — which 
are  resultant  from  the  developmental 
lobes  previously  composing  the  primitive 
cutting-edge,  and,  for  this  reason,  are 
otherwise  known  as  developmental 
grooves.  In  many  instances  one  or 
more  transverse  ridges  are  found  upon 
the  cervical  portion,  but  these  are 
supplemental  in  character. 

The  Lingual  Surface  of  the 
Crown  (Fig.  57). — This  surface  has  its 
borders  formed  by  three  marginal 
ridges  and  the  cutting-edge.  The 
marginal  ridges  are  pronounced  eleva- 
tions of  enamel,  and  surround  the 
surface  upon  three  sides,  the  intervening  space  in  many  instances 
being  a  decided  concavity  or  fossa — the  lingual  fossa.  The  mesial 
marginal  ridge  begins  at  the  mesial  angle  of  the  crown,  passes  upward, 
inward  and  backward,  following  the  curvature  of  the  mesial  border.  The 
distal  marginal  ridge  begins  at  the  distal  angle  of  the  crown  in  a  somewhat 
less  pronounced  form,  passes  upward,  backward,  and  inward,  following 
the  curvature  of  the  distal  surface.  Upon  reaching  the  cervical  portion 
of  the  crown  these  two  margins  unite  and  form  the  cervicomar  ginal  ridge. 
This  ridge  may  be  bold  and  prominent,  or  it  may  be  but  slightly  developed. 
Near  its  center  it  is  frequently  broken  by  a  depression  or  pit — the  lingual 
pit.  In  some  instances  this  pit  is  deeply  penetrating;  in  others  it  assumes 
the  form  of  a  fissure,  and  may  completely  sever  the  ridge.  This  border 
is  sometimes  elevated  into  a  sightly  developed  tubercle  or  cusp — the 
cuspule.  When  this  is  present  it  has  the  appearance  of  being  produced 
by  a  fold  of  enamel,   and  encircling  it  is  a  well-marked  fissure.     The 


■  >  1 


Mesial 
Marginal 
Ridge 


Mesial 
Angle 


Lingual  Fossa 

Fig.  57. — Upper  Central  Incisor, 

Lingual  Surface. 


UPPER    CENTRAL    INCISOR 


I05 


lingual  fossa  is  usually  traversed  by  two  longitudinal  grooves,  which 
correspond  to  the  developmental  grooves  of  the  labial  surface.  When 
a  capsule  is  present,  the  fissure  which  surrounds  it  frequently  passes 
into  the  fossa,  or  the  fossa  may  be  partly  covered  by  the  cuspule  over- 
hanging its  cervical  portion.  When  the  cervicolingual  fissure  exists,  it  is 
not  unusual  for  it  to  bifurcate  and  throw  a  branch  along  the  inner  border 
of  each  marginal  ridge,  or  it  may  penetrate  the  fossa  proper  and  divide 
it  into  two  parts.  The  lingual  surface  is  somewhat  less  in  extent  than  the 
labial  surface,  this  reduction  being  principally  in  a  mediodistal  direction, 
the  length  of  the  two  surfaces  from  the 
cutting-edge  to  the  cervical  margin 
being  about  equal. 

The  Mesial  Surface  of  the  Crown 
(Fig.  58). — The  outline  of  this  surface 
resembles  an  inverted  cone  or  triangle, 
the  lines  of  which  are  more  or  less 
broken,  the  apex  of  the  cone  terminat- 
ing at  the  cutting-edge  and  the  base 
directed  toward  the  root  of  the  tooth. 
The  base  of  the  cone  is  made  concave 
by  the  enamel  margin  or  cervical  line. 
The  margins  of  the  mesial  surface  are 
the  labial,  the  lingual,  and  the  cervical. 
The  labial  margin  is  convex  and 
rounded  throughout  its  entire  extent, 

from  the  cutting-edge  to  the  cervical  line.  The  contour  of  this  margin 
varies  with  the  typal  form  of  the  crown,  in  some  presenting  a  decided 
and  well-marked  convexity,  in  others  being  but  slightly  curved.  The 
lingual  margin  is  concave  and  rounded,  but  the  line  is  much  broken. 
Beginning  at  the  cutting-edge,  it  is  decided  and  square,  this  feature 
usually  including  the  lower  third.  As  it  passes  upward  and  the  center 
is  approached,  the  line  is  more  concave  and  rounded  in  a  mesiolingual 
direction,  this  latter  feature  increasing  upon  approaching  the  cervical 
line.  The  cervical  margin  is  that  formed  by  the  cervical  line.  It  is 
usually  well  defined,  being  concave  or  V-shaped,  with  the  point  of  the  V 
more  or  less  rounded,  and  with  its  free  ends  pointing  one  in  a  labial  and 
one  in  a  lingual  direction,  the  former  being  a  trifle  longer  than  the  latter. 
The  surface  between  the  borders  presents  a  slight  general  convexity, 
but  with  an  inclination  to  flatness  near  the  cervical  portion,  which  is 
occasionally   developed  into   a   slight   concavity.     Whatever   deviations 


Fig.  58. — Upper  Central  Incisor, 
Mesial  Surface. 


io6 


ANATOMY 


Lingual 

Fossa 

■1  wk 

Lingual     1 

Wk  jb 

Ridge 

Cervical  Line 

Cervical 

Ridge 


Fig.  59. — Upper  Central  Incisor, 
Distal  Surface. 


may  be  present  in  the  borders  of  this  surface,  from  those  assumed  in  the 

description  just  given,  their  union  at  the  cutting-edge  will  aways  be  in 

a  direct  line  with  the  long  axis  of 
the  tooth. 

The  Distal  Surface  of  the 
Crown  (Fig.  59). — In  a  general  way, 
this  surface  resembles  the  mesial 
surface  just  described.  There  are, 
however,  one  or  two  minor  points 
of  distinction:  the  borders  are  all 
more  rounded,  the  labial  border 
presenting  a  greater  convexity,  and 
the  lingual  a  more  perfectly  formed 
Distal  Angle  concavity.  The  surface  is  quite 
full  in  the  center,  from  which  it 
slopes  away  in  all  directions,  thus 
producing  a  decided  general  con- 
vexity.    The  cervical  margin  of  the 

surface  is  almost  identical  with  the  cervical  margin  of  the  mesial  surface. 

The  distance  in  a  direct  line  from  the  cervical  border  to  the  cutting-edge 

is  a  trifle  less  than  the  corresponding  measure- 
ment on  the  mesial  surface.     The  distal  angle 

is  equally  constant  in  its  position,  and,  being 

connected  with  the  mesial  angle  in  a  direct 

line  by  the  cutting-edge,  finds  this  latter  margin 

always  in  the  labiolingual  center  of  the  crown. 
The  Cutting-edge  of  the  Central  Incisor. — 

The  cutting  or  incisive  edge  receives  its  name 

from  its  function,  that  of  cutting  or  incising 

the  food.     It  is  formed  by  the  junction  of  the 

labial  and  lingual  surfaces  of  the  crown,  and 

extends  almost  in  a  direct  line  from  the  mesial 

to  the  distal  surface;  at  its  union  with  the 

mesial  surface  it  assists  in  forming  the  mesial 

angle  of  the  crown,  and  serves  the  same  pur- 
pose by  its  union  with  the  distal  surface.     In 

the  majority  of  instances  it  is  an  unbroken 

line.     In  passing  from  the  mesial  to  the  distal 

angle  it  converges  slightly  in  the  direction  of  the  root,  thus  making  the 

crown  a  trifle  shorter  on  the  mesial  than  on  the  distal  side.     In  the 


Fig.  60. — A  Young  Upper 
Central  Incisor,  Labial 
Surface,  showing  Develop- 
mental Grooves — a 


UPPER   CENTRAL   INCISOR  107 

recently  erupted  tooth  (Fig.  60)  the  line  is  broken  by  the  developmental 
grooves;  these  usually  disappear  by  wear,  but  occasionally  traces  of  their 
existence  remain,  and  thus  permanently  break  the  positive  line  that 
would  otherwise  be  present.  As  the  cutting-edge  approaches  the  distal 
angle  of  the  crown  it  is  inclined  to  slope  away,  producing  a  less  positive 
angle  than  the  corresponding  mesial  angle.  In  some  instances  the 
cutting-edge  is  quite  thin  and  inclined  to  sharpness,  in  others  it  is 
blunt  and  dull,  the  former  condition  being  present  when  there  is  a 
decided  overbite  in  occlusion,  the  latter  occurring  when  this  feature  is 
less  pronounced.  The  cutting-edge  is  frequently  referred  to  as  the 
occlusal  surface,  this  term  being  employed  to  make  the  description  more 
uniform  with  the  bicuspids  and  molars,  and  for  this  reason  is  permissi- 
ble; but  it  is  only  in  rare  instances  that  the  surfaces  occlude  directly  with 
the  opposing  teeth,  the  condition  being  most  frequent  in  teeth  of  the 
lymphatic  type,  and  in  cases  of  malocclusion. 

The  Cervical  Margin. — This  margin,  which  is  distinctly  outlined  by 
the  free  extremity  of  the  enamel  covering  of  the  crown,  also  marks 
the  extent  of  the  membrane  covering  the  root.  The  margins  formed  by 
this  line  are  those  of  a  double  concavity  and  a  double  convexity.  On 
the  labial  and  lingual  portions  it  is  concave  rootward,  while  on  the 
mesial  and  distal  sides  it  is  convex  in  this  direction.  If  a  line  be  drawn 
around  the  tooth  at  the  extreme  upper  point  of  the  enamel  covering, 
it  will  be  found  to  touch  only  the  labial  and  lingual  prolongations,  while 
a  space  will  exist  between  the  line  thus  drawn  and  the  cervical  margins 
of  the  proximate  surfaces.  The  character  of  the  cervical  curvature 
varies  with  the  type  of  the  tooth,  being  more  or  less  pronounced  as  the 
case  may  be.  In  a  typical  central  incisor  the  cervical  line  of  the  labial 
surface  will  usually  form  the  segment  of  a  larger  circle  than  that  of  the 
lingual,  and,  while  the  mesial  convexity  may  be  gracefully  curved,  the 
distal  may  incline  to  angularity. 

The  Neck  of  the  Tooth. — The  neck  of  this  tooth  partakes  of  a  form 
between  that  of  the  crown  and  root  which  it  joins.  It  is  principally 
formed  by  a  sudden  sloping  of  the  enamel  margin  to  meet  the  root.  It 
is  broader  on  the  labial  than  on  the  lingual  surface,  and  is  somewhat 
flattened  laterally,  with  an  occasional  depression  or  concavity  on  its 
mesial  portion.  The  neck  of  this  tooth  is  seldom  a  decided  anatomic 
feature,  being  less  pronounced  than  upon  any  other  tooth.  In  the  bicus- 
pids and  molars  both  the  crowns  and  the  roots  assist  in  forming  the  neck 
by  a  constriction  of  their  adjacent  parts,  while  in  this  tooth  the  crown 
alone  is  instrumental  in  this  direction. 


108  ANATOMY 

The  Root  of  the  Upper  Central  Incisor. — The  root  of  this  tooth 
is  conic  in  form,  its  base  directed  downward,  its  apex  upward.  Viewed 
in  transverse  sections  its  outline  is  that  of  a  rounded  triangle,  one  side 
of  which  faces  in  a  labial,  one  in  a  mesiolingual,  and  one  in  a  distolingual 
direction.  The  labial  side  is  the  most  flattened,  while  the  two  remaining 
sides  are  of  equal  length  and  oval  in  form.  This  triangular  outline 
usually  continues  throughout  the  entire  length  of  the  root,  but  in  some 
instances,  near  the  apical  end,  may  have  a  decided  or  slight  distal  curve, 
included  in  which  will  be  a  more  circular  form.  The  taper  of  the  root 
from  the  base  to  the  apex  is  very  gradual  upon  the  labial  and  lingual 
surfaces,  until  the  apical  third  is  reached,  when  the  two  sides  converge 
more  rapidly.  The  mesial  and  distal  surfaces  are  somewhat  flattened 
and  taper  very  gradually  from  the  base  to  the  apex.  In  a  majority  of 
instances  the  root  is  much  longer  than  the  crown,  but  in  rare  cases  its 
length  is  barely  equal  to,  or  less  than,  that  of  the  crown. 

Bilious  Type. — The  crown  of  the  upper  central  incisor  in  this 
type  is  of  greater  longitudinal  than  transverse  extent;  large  in  size,  abound- 
ing in  angles  rather  than  curves.  It  possesses  neither  brilliancy  nor 
transparency  of  surface,  but  is  slightly  inclined  to  translucent .  The 
labial  surface  is  flat,  with  more  or  less  decided  transverse  ridges  in  the 
cervical  portion.  The  labial  grooves  are  generally  present  in  the  form 
of  well-defined  depressions.  On  account  of  the  angular  nature  of  the 
tooth,  this  surface  approaches  closely  to  the  quadrilateral  form.  The 
mesial  and  distal  surfaces  are  flat,  with  their  margins  bold  and  well 
defined.  The  lingual  surface  also  shows  the  angular  nature  of  the  crown 
in  having  its  marginal  ridges  squarely  set  and  its  developmental  grooves 
definitely  outlined.  The  cutting-edge  is  rather  thin,  square,  and  sharp, 
the  line  frequently  being  imperfectly  formed.  The  mesial  and  distal 
angles  are  both  well  produced,  and  the  cervical  margin,  in  keeping  with 
the  rest  of  the  parts,  is  inclined  to  angularity. 

.  Nervous  Type. — The  central  incisor  common  to  this  temperament 
is  delicate  and  graceful  in  outline.  The  crown  is  of  medium  size,  with 
the  length  predominating  over  breadth.  The  enamel  is  inclined  to  trans- 
parency, and  is  of  a  blue  or  bluish-gray  color,  presenting  much  brilliancy. 
The  labial  surface  is  fairly  well  rounded,  and  the  labial  grooves  are 
present  as  slightly  rounded  depressions,  which  frequently  extend  well 
toward  the  cervical  margin,  where  they  gradually  disappear.  In  general 
outline  this  surface  partakes  of  the  triangular  form,  the  crown  of  the 
tooth  being  broad  at  the  cutting-edge  and  much  constricted  at  the  neck. 
The  mesial  and  distal  surfaces  show  a  convexity  in  every  direction, 


UPPER    CENTRAL    INCISOR  IO9 

and  the  nature  of  the  occlusion  is  manifest  from  the  decided  wedge- 
shape  appearance  of  the  crown  providing  for  a  long  overbite.  Upon 
the  lingual  surface  but  little  in  the  way  of  detail  is  to  be  observed,  the 
entire  surface  from  the  cutting-edge  to  the  cervical  ridge  being  smooth 
and  concave.  The  cuspule  previously  referred  to  is  occasionally  present 
in  this  type,  breaking  the  general  smoothness  of  the  surface  with  its 
prominence.  The  marginal  ridges  are  poorly  defined;  the  cutting-edge 
is  a  sharp,  unbroken  line;  the  mesial  and  distal  angles  are  present  in 
the  form  of  long,  graceful  curves,  rather  than  definite  angles,  this  being 
particularly  true  of  the  distal  angle.  The  cervical  line  is  decidedly 
curved,  the  labial  and  lingual  portions  being  deeply  concave  rootward, 
while  the  mesial  and  distal  are  decidedly  convex. 

Sanguineous  Type. — The  crown  of  the  central  incisor  is  usually 
above  the  average  in  size,  but  is  well  proportioned,  abounding  in  curves 
and  rounded  outlines.  The  enamel  is  inclined  to  translucency,  particu- 
larly near  the  cutting-edge.  The  labial  surface  is  smooth  and  rounded; 
the  depressions  formed  by  the  labial  grooves  are  slightly  observable, 
and  extend  but  a  short  distance  from  the  cutting-edge.  The  surface 
is  somewhat  greater  in  longitudinal  than  in  transverse  extent,  and  ap- 
proaches much  nearer  to  a  circular  form  than  the  corresponding  tooth 
of  other  types.  The  mesial  and  distal  surfaces  are  well  rounded,  making 
the  point  of  contact  with  approximating  teeth  near  the  center  of  the 
surface.  The  lingual  surface  abounds  in  heavy  rounded  lines;  the 
marginal  and  cervical  ridges  are  particularly  prominent,  diminishing 
the  extent  of  the  lingual  fossa.  A  cuspule  is  frequently  present  in  the 
form  of  a  well-rounded  prominence.  The  cutting-edge  is  of  moderate 
thickness  and  slopes  away  from  the  center  in  either  direction  to  assist 
in  forming  the  rounded  mesial  and  distal  angles.  The  cervical  curvature 
on  the  labial  and  lingual  surfaces  is  an  unbroken  semicircle,  while  that 
of  the  mesial  and  distal  surfaces  is  less  uniform. 

Lymphatic  Type. — In  the  central  incisor  of  this  typal  form  the 
crown  is  large,  but  not  shapely,  and  the  breadth  is  equal  to,  or  exceeds, 
the  length.  The  enamel  coloring  is  muddy  or  brownish-yellow,  and  the 
surface  is  lacking  in  brilliancy.  The  labial  surface  is  flat  and  smooth, 
with  a  faint  sign  of  the  labial  grooves.  The  general  outline  of  this 
surface  is  that  of  a  circular  cone,  with  the  cutting-edge  for  the  base,  and 
the  apex  formed  on  the  cervical  margin.  The  mesial  and  distal  aspects 
present  a  striking  contrast  to  the  types  previously  described,  by  having 
a  labiolingual  diameter  greater  than  that  represented  between  the  cervical 
line  and  the  cutting-edge.     These  two  surfaces  are  convex  in  a  labio- 


I IO  ANATOMY 

lingual  direction  only,  making  the  point  of  contact  with  approximating 
teeth  an  extended  surface  rather  than  a  single  point. 

The  lingual  surface  is  heavy  and  bulky,  frequently  to  such  a  degree 
as  to  produce  a  general  convexity  rather  than  a  concavity,  as  found  in 
most  typal  forms.  This  surface  is  frequently  broken  by  one  or  more 
longitudinal  grooves,  but  is  seldom  crossed  by  transverse  lines  of  any 
kind.  The  cutting-edge  is  barely  deserving  of  the  name.  Although 
formed  by  the  free  borders  of  the  labial  and  lingual  surfaces,  these  two 
planes  are  so  far  separated  at  their  incisive  margins  that  the  space  between 
them,  instead  of  being  an  edge,  becomes  a  more  or  less  broadened  surface, 
and  one  upon  which  the  lower  incisors  frequently  occlude.  The  line 
thus  formed  is  straight  and  direct  from  the  mesial  to  the  distal  angle 
of  the  crown,  both  of  which  are  well  produced.  The  cervical  curvature 
is  represented  by  the  segment  of  a  much  larger  circle  than  that  found 
upon  teeth  of  other  types,  and  the  neck  of  the  tooth  is  heavy  and  bulky, 
showing  but  little  constriction  at  this  point. 

UPPER  LATERAL  INCISOR. 


4th 

5th 

7  th 

9th 

ioth 

nth 

year 

year 

year 

year 
Fig.  6i. 

year 

year 

Calcification  Begins,  from  Three  Centers,  First  Year  after  Birth. 
Calcification  Completed,  Tenth  to  Eleventh  Year. 
Erupted,  Seventh  to  Eighth  Year. 
Average  Length  of  Crown,  .34. 

Average  Length  of  Root,  .51. 

Average  Length  over  All,  .85. 

Like  the  central  incisor,  calcification  in  this  tooth  begins  during 
the  first  year  after  birth,  the  process  taking  place  in  the  same  manner, 
from  three  centers,  along  the  future  cutting-edge,  and  gradually  extend- 
ing in  the  direction  of  the  root.  By  the  expiration  of  the  third  year  the 
cutting-edge  and  the  angles  of  the  crown  are  fully  formed;  the  fourth 
year  finds  the  crown  calcified  to  nearly  one-half  its  completed  length; 


UPPER    LATERAL   INCISOR 


III 


by  the  fifth  year  the  cervical  ridge  is  reached;  while  the  sixth  year  usually 
completes  the  process  of  calcification  in  the  crown.  At  the  close  of  the 
seventh  year  the  base  of  the  root  is  fully  outlined,  during  the  following  year 
about  one-eighth  of  an  inch  is  added  to  its  length,  and  still  greater  progress 
is  made  during  the  ninth  year,  by  which  time  fully  three-fourths  of  the 
root  length  has  become  calcified.  During  the  tenth  year  the  apical  end 
of  the  root  begins  to  form  by  a  sudden  doubling-over  of  the  free  cal- 
cifying margins,  and  by  the  eleventh  year  the  surface  of  the  root  is  com- 
plete (Fig.  61).  By  the  above  description  it  will  be  observed  that  at  the 
time  of  eruption  the  root  of  this  tooth  is  only  calcified  to  about  one-half 
of  its  completed  length,  and  the  same  may  be  said  of  the  central  incisor; 
but  so  much  time  elapses  between  the  beginning  of  the  eruptive  stage 
and  the  period  at  which  this  phenomenon  is  completed  that  the  calcifica- 
tion of  the  root  is  usually  finished  by  the 
time  the  tooth  assumes  its  permanent 
position  in  the  jaw. 

The  crown  of  the  upper  lateral 
incisor,  like  that  of  the  upper  central 
incisor,  presents  for  examination  four 
surfaces — labial,  lingual,  mesial,  and 
distal — a  cervical  margin,  a  cutting-edge, 
and  a  mesial  and  distal  angle.  The 
general  contour  of  the  crown  closely  re- 
sembles that  of  the  upper  central  incisor, 
except  that  it  measures  about  one-third 
less  from  mesial  to  distal,  and  is  a  trifle 
shorter  from  the  cutting-edge  to  the 
cervical  line.  As  in  the  central  incisor, 
the  labial  and  distal  surfaces  form  a 
double  incline  plane,  and  unite  below  to 
form  the  cutting-edge.  The  labial  side  of  the  incline  is  convex,  while 
the  lingual  is  concave,  but  seldom  so  marked  as  that  of  the  central  incisor. 
The  base  of  the  wedge,  or  double  incline,  formed  by  the  cervical  margin, 
is  correspondingly  smaller  than  that  of  the  crown  of  the  central  incisor. 

The  Labial  Surface  of  the  Crown  (Fig.  62). — This  surface  of  the 
crown  of  the  upper  lateral  incisor  is  more  irregular  in  outline  than  the 
corresponding  surface  of  the  central  incisor.  The  margins  of  the  surface 
are  the  mesial  distal,  cervical,  and  incisive.  The  mesial  margin  begins 
at  the  mesial  angle  and  passes  upward  with  a  decided  distal  inclination 
to  meet  the  cervical  margin.     The  distal  margin  is  shorter  and  decidedly 


Labial  Grooves 

Fig.  62. — Upper  Lateral  Incisor, 
Labial  Surface. 


112 


ANATOMY 


Cervical 
Ridge 

Distal  Mar- 


more  convex  than  the  mesial  margin,  this  variation  in  outline  being  still 
more  marked  when  compared  with  the  corresponding  margin  of  the 
central  incisor.  At  the  cutting-edge  these  two  margins  assist  in  forming 
the  mesial  and  distal  angles  of  the  crown,  and  by  their  continuation  and 
union  above  form  the  cervical  margin.  The  incisive  margin  is  formed 
by  the  cutting-edge.  Like  the  central  incisor,  the  four  margins  of  this 
surface  vary  greatly  in  the  different  types;  this  is  particularly  true  of 
the  two  lateral  margins,  which  at  times  are  found  to  be  in  the  form  of  a 

direct  line,  or  even  slightly  concave, 
while  in  others  they  are  both  decid- 
edly convex.  This  surface  of  the 
crown  shows  a  greater  general  con- 
vexity than  the  labial  surface  of  the 
central  incisor,  the  cervical  portion 
presenting  a  curve  much  more  de- 
cided than  that  near  the  cutting- 
edge.  The  labial  grooves  are  in  all 
respects  similar  to  those  described  in 
ge  connection  with  the  central  incisor, 
and  extend  from  the  cutting-edge  to- 
ward the  center  of  the  surface,  where 
they  gradually  disappear.  Trans- 
verse ridges  are  occasionally  found 
near  the  cervical  portion  of  the 
surface. 
The  Lingual  Surface  of  the  Crown  (Fig.  63). — This  surface  of 
the  upper  lateral  incisor  is  subject  to  much  variation  in  form,  but  presents 
the  same  points  for  examination  as  the  corresponding  surface  of  the 
central  incisor.  These  consist  of  the  marginal  ridges,  which  are  usually 
more  pronounced  than  those  of  the  central,  making  the  concavity  or 
fossa  between  them  small  and  deep.  In  some  instances  the  surface  will 
be  smooth  and  flat,  with  an  entire  absence  of  ridges  or  fossae.  The 
distal  marginal  ridge  is  shorter  and  more  bowed  than  the  mesial,  and  the 
cervical  ridge  is  well  marked  and  proportionately  broader  and  stronger 
than  in  the  central  incisor.  In  some  instances  the  marginal  ridges  are 
but  slightly  developed,  with  their  cervical  ends  broadened  and  separated 
by  a  deep  fissure,  giving  the  appearance  of  a  terminal  fold  in  the  enamel. 
The  cervical  ridge  is  frequently  broken  by  a  cuspule,  which  is  usually 
more  pronounced  than  when  found  upon  the  central  incisor.  The 
lingual  fossa  may  be  present  as  a  smooth,  unbroken  concavity,  or  it  may 


Langupl  Grooves 

Fig.  63. — Upper  Lateral  Incisor, 
Lingual  Surface. 


UPPER   LATERAL   INCISOR 


113 


Fig.  64. — Upper 
Lateral  Incisor, 
Mesial     Surface. 


be  subdivided  by  a  longitudinal  ridge,  which  often  exists  to  such  an 
extent  as  to  force  the  remaining  portions  of  the  fossa  well  against  the 
marginal  ridges,  where  they  will  be  observed  as  slight  depressions  rather 
than  marked  concavities. 

The  Mesial  Surface  of  the  Crown  (Fig.  64). 
— Viewing  the  crown  from  this  aspect,  the  outline 
is  that  of  an  inverted  cone  or  triangle.  The  lingual 
margin  of  the  surface  is  well  defined,  and  the  angle 
formed  by  the  union  of  this  surface  with  the  lingual 
surface  is  moderately  acute.  The  labial  margin  is 
well  rounded,  and  passes  into  the  labial  surface 
without  a  decided  line  of  demarcation.  The  sur- 
face on  its  upper  or  cervical  third  is  usually  flat- 
tened and  occasionally  concave.  At  the  center,  and 
continuing  toward  the  cutting-edge,  it  is  decidedly 
convex  in  every  direction,  thus  producing  a  promi- 
nent point  of  contact  with  the  distal  surface  of  the 
central  incisor. 

The  Distal  Surface  of  the  Crown  (Fig.  65). 
— This  surface  also  shows  the  characteristic  wedge-shape  of  the  crown, 
and  is  principally  different  from  the  mesial  surface  in  being  convex 
throughout.  Near  the  center  it  is  well  rounded  and  full,  providing  a- 
point  of  contact  for  the  mesial  surface  of  the  cuspid. 
The  lingual  margin,  while  being  more  decided  in  out- 
line than  the  labial,  is  much  more  rounded  than 
the  lingual  margin  of  the  mesial  surface.  From  the 
most  prominent  point  near  its  center  the  surface  slopes 
away  in  every  direction,  the  convexity  being  most 
marked  near  the  cutting-edge. 

The  Cutting-edge  of  the  Lateral  Incisor. — In  the 
young  tooth  the  cutting-edge  presents  the  three  little 
tubercles  common  to  all  incisors,  the  grooves  which 
divide  them  passing  up  over  the  labial  and  lingual 
surfaces  and  forming  the  labial  and  lingual  grooves. 
These  tubercles  soon  disappear  by  wear,  leaving 
the  cutting-edge  in  the  form  of  a  direct  lines  and 
connecting  the  two  angles  of  the  crown.  Like  the 
central  incisor,  this  margin  of  the  crown  may  be  thin  and  sharp,  or  it  may 
be  thick  and  dull. 

The  Cervical  Margin. — The  line  of  demarcation  between  the  crown 


Fig.  65.  —  Upper 
Lateral  Incisor, 
Distal  Surface. 


114  ANATOMY 

and  the  root  of  the  tooth  resembles  so  closely  that  described  in  connection 
with  the  central  incisor  that  it  will  only  be  necessary  to  mention  one  or 
two  characteristic  differences.  The  lingual  side  of  the  line  presents  a 
much  smaller  curve  proportionately,  and  usually  extends  a  little  higher 
in  the  direction  of  the  root  than  that  represented  upon  the  labial  portion. 
The  mesial  and  distal  portions  of  the  line  dip  well  down,  decreasing  the 
length  of  the  crown  on  these  surfaces;  the  margin  on  the  former  surface 
is  usually  angular  and  V-shaped,  while  on  the  latter  it  is  circular  in  form. 

The  Angles  of  the  Crown. — The  angles  of  the  crown  are  the  mesial 
and  the  distal,  and  are  formed  in  the  same  manner  as  the  same  angles  of 
the  central  incisor.  The  mesial  angle  is  generally  well  produced,  in  most 
instances  being  slightly  acute;  but  when  the  cutting-edge  is  thin  and 
frail,  the  angle  is  frequently  much  obliterated  by  wear.  That  portion 
of  the  crown  of  the  upper  lateral  incisor  which  is  usually  referred  to  as 
the  distal  angle  is  scarcely  worthy  of  the  name.  It  is  usually  present  as 
a  long  curve,  which  begins  near  the  center  of  the  cutting-edge  and  extends 
well  up  on  the  distal  surface.  This  characteristic  outline  is  sometimes 
so  pronounced  as  to  completely  destroy  the  cutting-edge,  the  distal 
surface  being  carried  forward  by  a  long  curve  ending  in  the  mesial  angle. 

The  Neck  of  the  Tooth. — In  this  tooth  the  neck  is  usually  marked 
by  a  constriction  much  more  pronounced  than  that  found  in  the  central 
incisor.  On  the  labial  and  lingual  surfaces  it  is  principally  formed  by 
a  sudden  sloping  of  the  enamel  surface  rootward,  but  on  the  two  lateral 
surfaces  it  is  formed  by  a  flattening  or  slight  concavity  of  both  the  crown 
and  the  root. 

The  Root  of  the  Upper  Lateral  Incisor. — The  root  of  this  tooth 
is  conic  in  form,  and  is  much  more  flattened  from  mesial  to  distal  than 
the  root  of  the  central  incisor.  At  its  junction  with  the  crown  it  is  circu- 
lar in  form,  the  labial  portion  forming  the  segment  of  a  larger  circle  than 
the  lingual,  this  feature  being  observed  throughout  its  entire  length.  The 
flattening  of  the  mesial  and  distal  sides  begins  immediately  above  the 
neck,  and  gradually  increases  as  the  center  of  the  root-length  is  approached, 
where  it  often  develops  into  a  slight  longitudinal  depression.  As  the 
apex  of  the  root  is  approached,  this  longitudinal  depression  gradually 
disappears,  and  the  root  again  becomes  circular  in  form.  The  thickness 
of  the  root  is  about  one-third  greater  from  labial  to  lingual  than  from 
mesial  to  distal,  and,  while  it  is  generally  classed  as  a  straight  root,  it  is 
frequently  provided  with  a  pronounced  distal  curve  near  the  apical 
extremity.  In  some  instances  it  is  found  with  a  double  mesiodistal 
curve. 


UPPER  LATERAL  INCISOR  115 

Bilious  Type. — In  this  type  the  lateral  incisor  is  frequently  poorly 
developed,  the  cutting-edge  and  distal  angle  are  wanting,  the  crown 
being  in  the  form  of  a  single  conic  cusp,  the  distal  surface  meeting  the 
mesial  at  a  point  near  the  mesial  angle.  This  form  of  crown  might 
be  classed  as  one  of  malformation,  but  the  fact  that  it  most  frequently 
occurs  in  this  temperament  would  appear  to  indicate  a  normal  condition. 
When  the  crown  takes  the  form  common  to  incisors,  it  is  of  greater 
longitudinal  than  transverse  extent,  the  angles  are  well  produced,  and 
the  mesial  and  distal  surfaces  are  flat  and  almost  parallel  with  each  other. 
The  labial  surface  is  flat  and  is  frequently  broken  by  transverse  ridges 
near  the  cervical  portion.  The  lingual  surface  presents  well-marked 
outlines  and  margins,  a  cuspule  is  seldom  present,  and  the  lingual  fossa 
is  well  marked,  but  not  deep.  The  cutting-edge  is  thin  and  sharp,  to 
provide  for  the  overbite,  which  is  rather  long.  The  cervical  border  is 
square  and  angular. 

Nervous  Type. — In  this  typal  form  the  neck  of  the  tooth  is  a  pro- 
nounced feature.  The  crown  is  long  and  narrow,  the  constriction 
forming  the  neck  beginning  well  down  on  the  crown  and  extending  over 
the  cervical  line  to  the  surface  of  the  root.  The  labial  surface  is  convex 
in  every  direction  and  the  labial  grooves  fairly  well  defined.  The  mesial 
surface  is  convex  near  the  center  and  cutting-edge,  but  often  shows  a 
slight  concavity  on  its  cervical  portion.  The  distal  surface  is  rounded 
and  smooth.  The  lingual  surface  presents  a  general  concavity,  a  cuspule 
being  more  frequently  present  than  in  other  types..  The  lingual  fossa 
is  deep,  and  often  extends  beneath  the  cervicomarginal  ridge  in  the  form 
of  a  circular  fissure.  The  cutting-edge  is  thin  and  sharp,  providing  for 
a  long  overbite;  the  mesial  angle  is  pointed  and  well  formed,  while  the 
distal  is  usually  much  rounded.  The  cervical  line  is  well  arched,  forming 
the  segment  of  a  much  smaller  circle  than  that  seen  on  the  same  tooth 
of  other  temperaments. 

Sanguineous  Type. — The  crown  is  well  proportioned,  with  the 
length  slightly  predominating  over  breadth,  all  the  surfaces  being  more 
or  less  rounded  and  smooth,  showing  the  crown  to  be  made  up  of  curves 
rather  than  angles.  The  labial  surface  presents  a  graceful  convexity 
throughout;  the  mesial  and  distal  surfaces  are  both  convex,  with  their 
margins  poorly  defined.  The  lingual  surface  shows  the  rounded  nature 
of  the  crown  in  having  its  fossa  and  marginal  ridges  oval  and  blending 
one  into  the  other.  The  cutting-edge  is  moderately  heavy  and  dull, 
in  keeping  with  the  overbite,  which  is  short.  The  cervical  line  is  made 
up  of  curves  rather  than  angles. 


u6 


ANATOMY 


Lymphatic  Type. — In  this  type  the  crown  is  generally  of  greater 
transverse  than  longitudinal  extent.  The  neck  is  poorly  produced,  the 
crown  and  root  uniting  without  any  marked  constriction  of  the  parts. 
The  labial  surface  is  much  flattened  from  mesial  to  distal,  and  but  slightly 
convex  in  the  direction  of  the  long  axis  of  the  tooth.  The  mesial  and 
distal  surfaces  are  but  little  rounded  and  are  nearly  parallel  with  each 
other,  so  that  the  contact  with  adjoining  teeth  becomes  an  extent  of 
surface  rather  than  a  single  point.  The  lingual  surface  is  convex  above, 
but  as  the  cutting-edge  is  approached  it  becomes  flat,  but  seldom  con- 
cave. The  marginal  ridges  are  not  well  shown  and  the  lingual  fossa  is 
but  a  slight  depression.  The  angles  of  the  crown  are  well  produced  and 
the  cutting-edge  thick  and  blunt,  this  marginal  surface  frequently  occlud- 
ing directly  upon  the  opposing  lower  teeth.  The  curvature  of  the  cer- 
vical line  is  that  of  a  long  circle. 

UPPER  CUSPID. 


5  th 

6  th 

7th 

8th 

8th 

ioth 

13th 

year 

year 

year 

year 
Fig.  66. 

year 

year 

year 

Calcification  Begins,  from  Three  Centers,  Third  Year  after  Birth. 
Calcification  Completed,  Twelfth  to  Thirteenth  Year. 
Erupted,  Twelfth  to  Thirteenth  Year. 
Average  Length  of  Crown,  .37. 

Average  Length  of  Root,  .68. 

Average  Length  over  All,  1.05. 


About  the  third  year  after  birth  calcification  begins  in  the  central 
lobe,  which  is  gradually  extended  laterally,  until,  at  the  fourth  year 
it  is  met  by  the  two  lateral  lobes,  which  are  somewhat  later  in  beginning, 
and  by  the  fifth  year  the  three  are  united,  the  former  eventually  establish- 
ing the  single  cusp  of  the  tooth  and  the  latter  two  the  mesial  and  distal 


UPPER    CUSPID 


117 


angles.  About  the  sixth  year  two-thirds  of  the  crown  is  formed,  and  by 
the  seventh  year  the  constriction  which  marks  the  beginning  of  the  neck 
of  the  tooth  commences  to  make  its  appearance.  Between  the  seventh 
and  eighth  year  calcification  in  the  crown  is  completed  and  the  cervical 
line  established,  during  the  following  year  nearly  one-quarter  of  an  inch 
is  added  to  the  length  of  the  root,  and  by  the  beginning  of  the  tenth  year 
the  root  is  formed  for  fully  two-thirds  of  its  entire  length.  Between  the 
twelfth  and  thirteenth  years  or  at  the  time  of  eruption,  calcification  is 
completed  in  the  root  so  far  at  its  surface  is  concerned  (Fig.  66).  In 
this  latter  particular  the  cuspid  differs  from  most  of  the  other  teeth,  in 
being  completely  calcined  previous  to,  or  about  the  time  of,  its  eruption. 
To  reach  its  final  position  in  the  arch  the  tooth  moves  bodily  downward, 
the  bone  filling  in  behind;  while  in  the  incisors,  bicuspids,  and  molars 
the  free  calcifying  root-extremi- 
ties remain  nearly  stationary,  the 
crowns  being  forced  downward  as 
the  lime  salts  are  deposited. 

The  crown  of  the  upper 
cuspid  presents  for  examination 
four  surfaces — labial,  lingual, 
mesial,  and  distal — two  margins 
— the  cervical  margin  and  the 
cutting-edge — and  a  mesial  and 
a  distal  angle.  In  general  out- 
line it  is  of  the  simplest  form, 
resembling  the  primitive  cone- 
shaped  teeth  of  many  fishes. 
When  viewed  by  looking  directly 
upon  the  mesial  or  distal  surface, 
the  wedge-shape  common  to  the 
incisors  is  observed.     The  base 

of  the  double  incline  is,  however,  much  broader  proportionately  than  the 
corresponding  measurement  of  the  incisors.  Looking  at  the  crown  from 
a  labial  or  lingual  direction,  its  function,  as  both  a  penetrating  and 
incising  organ,  may  be  observed  in  the  single  cusp  from  which  it  derives 
its  name.  The  cusp,  which  is  formed  at  the  expense  of  the  cutting- 
edge,  divides  this  latter  margin  into  two  distinct  portions — the  mesial 
cutting-edge  and  the  distal  cutting-edge. 

The  Labial  Surface   of  the  Crown   (Fig.   67).— The  contour  of 
this  surface  is  that  of  a  broken  circle  more  or  less  imperfectly  drawn.     It 


Fig.  67. 


Labial  Ridge 
-Upper  Cuspid,  Labial  Surface. 


n8 


ANATOMY 


is  bounded  by  five  margins — mesial,  distal,  cervical,  mesial-incisive,  and 
distal-incisive.  The  mesial  margin  is  rounded  from  labial  to  mesial,  and 
slightly  convex  from  the  cutting-edge  to  the  cervical  line.  The  distal 
margin  is  also  rounded  from  labial  to  distal,  presents  a  greater  convexity, 
and  is  somewhat  shorter  from  the  cutting-edge  to  the  cervical  line  than 
the  mesial  margin.  By  a  continuation  and  final  union  of  these  two 
lateral  margins  the  cervical  margin  of  the  surface  is  formed,  while  by 
their  union  with  the  cutting-edges  the  mesial  and  distal  angles  of  the 
crown  are  established.     The  mesial-incisive  margin  is  usually  slightly 

concave  near  its  center,  although  in 
some  instances  it  is  convex.  The 
distal  incisive  margin  responds  to  the 
same  description,  although  the  con- 
cavity, when  present,  is  nearest  the 
point  of  the  cusp.  From  the  summit 
of  the  cusp  these  two  margins  slope 
away  to  join  the  mesial  and  distal 
angles,  the  distal  incline  being  about 
one-fourth  longer  than  the  mesial. 
This  surface  is  generally  of  greater 
longitudinal  than  transverse  extent, 
its  greatest  mesiodistal  diameter  being 
from  angle  to  angle,  or  at  a  point 
immediately  above  them.  The  sur- 
face is  convex  in  every  direction,  and 
is  marked  by  a  central  longitudinal 
ridge,  usually  well  defined — the  labial 
ridge.  Beginning  at  the  summit  of  the  cusp,  this  ridge  is  more  or  less 
contracted  laterally,  but  as  it  passes  over  the  surface  in  the  direction  of 
the  root  it  becomes  broadened  and  flattened,  and  gradually  disappears 
in  the  cervical  portion.  Upon  either  side  of  this  ridge  are  the  labial 
grooves,  well  defined  at  their  beginning,  but  which  gradually  blend  into 
the  surface  of  the  crown  as  they  pass  rootward.  In  some  instances  these 
grooves  are  so  strongly  defined  as  to  form  a  decided  ridge  upon  the  mesial 
and  distal  margins  of  the  surface — these  are  the  labial  marginal  ridges. 

The  labial  ridge  and  the  two  labial  grooves  mark  the  developmental 
lines  of  the  crown,  the  former  resulting  from  the  middle  lobe,  which 
in  this  tooth  is  much  the  largest  of  the  three,  while  the  latter  denotes 
the  line  of  junction  between  the  middle  and  the  lateral  lobes. 

The  Lingual  Surface  of  the  Crown  (Fig.  68). — This  surface  pre- 


Lingual  Ridge 
Fig.  68. — Upper  Cuspid,  Lingual  Surface. 


UPPER    CUSPID 


119 


sents  nearly  the  same  general  outlines  as  the  labial,  with  the  exception 
of  the  cervical  portion,  which  is  more  constricted,  tending  to  produce  an 
oblong  or  egg-shape.  It  usually  abounds  in  well-defined  ridges  and 
depressions,  giving  to  the  tooth  a  rugged  and  strong  appearance.  There 
is  but  little  general  concavity  to  the  surface  in  passing  from  the  point  of 
the  cusp  to  the  root.  It  may  be  flat,  concave,  or  convex.  As  in  the 
incisors,  the  margins  of  this  surface  are  formed  by  three  marginal  ridges 
and  by  the  cutting-edge.  The  mesiomarginal  ridge  is  commonly  a  well- 
defined  fold  of  enamel,  beginning  at  the  mesial  angle  and  passing  upward 
in  the  direction  of  the  root,  where  it 
unites  with  the  cervicomarginal  ridge. 
It  is  sometimes  quite  narrow  and  rather 
sharply  outlined;  at  others,  it  extends 
well  toward  the  center  of  the  surface  in 
the  form  of  a  well-rounded  fold.  The 
distomarginal  ridge,  which  is  some- 
what shorter  than  the  mesial,  begins  at 
the  distal  angle  and  passes  rootward  to 
meet  the  cervical  ridge.  It  is  well 
rounded  in  every  direction,  but  seldom 
so  well  produced  as  the  mesial.  The 
cervicomarginal  ridge,  which  is  formed 
by  a  continuation  or  union  of  the  two 
former,  nearly  always  partakes  of  their 
nature,  except  when  broken  by  the 
presence  of  a  cuspule,  which  is  fre- 
quently found  upon  this  tooth  (Fig.  72). 

This  small  cusp  of  enamel  may  be  bounded  on  one  or  both  sides  by  a 
fissure,  which  often  extends  well  under  the  cervicomarginal  ridge,  and 
sometimes  completely  separates  it  from  the  two  lateral  ridges.  Passing 
through  the  center  of  the  surface  from  the  summit  of  the  cusp  to  the  base 
of  the  cervicomarginal  ridge  is  the  lingual  ridge,  which  corresponds  to 
the  labial  ridge  of  the  labial  surface.  This  ridge  is  usually  well  pro- 
duced at  or  near  the  point  of  the  cusp,  and  may  continue  so  throughout, 
but  most  frequently  becomes  reduced  in  size  near  the  center  of  the  sur- 
face. Between  this  ridge  and  the  mesio-  and  disto-marginal  ridges  are 
two  longitudinal  depressions — the  lingual  grooves. 

Mesial  Surface  of  the  Crown  (Fig.  69). — In  general  outline  this 
surface  resembles  that  of  the  central  incisor,  excepting  that  the  wedge- 
shape  which  it  describes  is  more  heavily  set  and  blunt,  with  the  surface 


w 

.  kJ5 

''H 

CeovicaJ 

Ridge 

Labia) 

Groove 

L    M 

Cervico- 
lingual 
Ridge 


Lingual 
Ridge 


Fig.  69.- 


-Right  Upper  Cuspid,  Mesial 
Surface. 


120 


A  \.\H>  MY 


extending  beyond  the  base  of  the  cone,  in  the  direction  of  the  root,  to 
the  extent  of  about  one-third  of  its  entire  length.  In  some  cases  the  base 
of  the  cone  will  be  on  a  line  with  the  cervical  margin.  The  lower  two- 
thirds  of  the  surface,  or  that  nearest  the  mesial  angle,  is  convex  in  every 
direction;  this  convexity  gradually  disappears  as  the  center  is  approached, 
beyond  which  point  it  is  much  flattened,  usually  ending  in  a  slight  con- 
cavity at  the  cervical  margin.  When  looking  directly  upon  this  surface, 
its  margins  will  be  found  within  the  profile  lines,  these  being  represented 
by  the  labial  ridge  anteriorly,  and  by  the  lingual  and  cervical   ridges 

posteriorly.  The  margins, 
three  in  number,  are  the 
labial,  which  is  well  rounded 
and  poorly  denned;  the  lin- 
gual, more  or  less  distinctly 
outlined  and  somewhat  irregu- 
lar; and  the  cervical,  which 
is  represented  by  the  extent  of 
the  enamel  covering  of  the 
crown;  this  latter  margin  be- 

|Cervicohn-  ° 

jguai  Ridge  mg  concave  in  the  direction  of 
the  root.  The  most  promi- 
nent point  of  this  surface 
serves  as  a  point  of  contact 
for  the  distal  surface  of  the 
lateral  incisor,  the  extent  of 
contact  being  much  influenced 
by  the  type  of  tooth,  but  in 
the  cuspid  this  is  usually  a 
single  point  rather  that  an  extent  of  surface. 

The  Distal  Surface  of  the  Crown  (Fig.  70). — This  surface  in 
many  respects  is  similar  to  the  mesial,  particularly  in  its  general  outline. 
The  extent  of  surface  is  somewhat  less  and  the  convexity  much  more 
marked  than  that  of  the  mesial  surface.  The  position  of  the  distal 
angle,  which  is  the  lower  boundary  of  the  surface,  being  much  nearer 
the  cervical  line,  makes  this  surface  about  one-third  shorter  than  the 
mesial  surface.  The  lateral  margins  of  the  surface,  which  are  also  within 
the  profile  lines,  differ  from  those  of  the  mesial  in  being  more  clearly 
defined.  The  cervical  margin  differs  from  that  of  the  mesial  surface 
by  having  a  concavity  with  much  less  depth.  As  stated  above,  the  surface 
is  decidedly  more  convex  than  the  mesial,  the  point  of  contact  for  the 


Distal 
Angle 


Summit  of 
Cusp. 


Fig.  70. — Left  Upper  Cuspid,  Distal 
Surface. 


UPPER    CUSPID 


121 


mesial  surface  of  the  first  bicuspid  being  almost  in  the  center.  Near 
the  cervical  margin  the  surface  is  inclined  to  flatness,  and  frequently 
concave. 

The  Cutting-edge,  or  Cusp. — As  inferred  in  the  beginning  of  this 
description,  the  cuspid  is  both  an  incising  and  a  penetrating  organ, 
the  latter  function  being  provided  for  by  the  presence  of  the  single  cusp, 
which  divides  the  cutting-edge  into  an  anterior  or  mesial  portion  and  a 
posterior  or  distal  portion.  The  mesial  cutting-edge  begins  at  the  summit 
of  the  cusp  and  slopes  away  to  meet  the  mesial  angle,  which  it  assists 
in  forming.  The  outline  of  this  edge  is 
usually  gracefully  curved  and  unbroken 
unless  permanently  crossed  by  the  labial 
groove.  The  distal  cutting-edge  is  gen- 
erally somewhat  longer  than  the  mesial. 
Immediately  after  leaving  the  summit  of 
the  cusp  it  may  be  slightly  concave;  but 
beyond  this  point  it  is  well  rounded, 
until  it  reaches  the  distal  angle,  into  cervicaj| 
which  it  gradually  disappears.  This 
edge  is  also  frequently  broken  by  the  ^£^1 
labial  groove.  In  its  entirety  the  cut- 
ting-edge is  subject 'to  the  same  varia- 
tions as  those  of  the  incisors — i.  e.,  it  may 
be  thin  and  sharp,  or  it  may  be  thick 
and  blunt. 

The  Cusp. — The  single  cusp  from  which  this  tooth  derives  its  name 
is  formed  by  the  union  of  the  labial  ridge,  the  lingual  ridge,  and  the 
mesial  and  distal  cutting-edges.  The  summit  of  the  cusp  is  constant 
in  its  position,  always  being  in  a  direct  line  with  the  long  axis  of  the 
tooth,  whether  it  be  viewed  from  a  mesial  or  a  lingual  direction. 

The  Cervical  Line. — To  describe  this  fully  would  be  to  repeat  what 
has  already  been  said  in  connection  with  the  incisors.  This  enamel 
margin  differs  in  one  particular  only  from  that  of  the  incisors,  and  that 
variation  is  not  a  constant  one — the  lingual  portion  is  frequently  extended 
in  the  direction  of  the  root,  producing  a  short,  positive  curve  at  that 
point. 

The  Angles  of  the  Crown. — Owing  to  the  rounded  nature  of  the 
majority  of  cuspid  crowns,  the  term  angle,  as  applied  to  its  free  extremities, 
is  almost  a  misnomer,  and  can  only  be  considered  as  assisting  in  descrip- 
tion.    The  mesial  angle,  which  is  formed  by  the  union  of  the  marginal 


Marginal  Ridges 
Fig.  71. — Upper  Cuspid,  Lingual 
Surface,  Strongly  Developed. 


122 


ANATOMY 


ridges  of  the  labial  and  lingual  surfaces  with  the  mesial  cutting-edge,  is 
seldom  a  well-produced  angle,  usually  being  rounded  in  every  direction. 
The  distal  angle,  which  is  formed  in  a  manner  similar  to  the  mesial,  is 
somewhat  more  deserving  of  the  name,  both  the  labio-  and  linguo-margi- 
nal  ridges  frequently  presenting  angularity.  The  position  of  the  distal 
angle  is  usually  well  toward  the  center  of  the  crown,  and  occasionally 
above  this  point,  and,  although  it  may  descend,  it  is  seldom  found  on 
a  line  with  the  mesial  angle. 

The  Neck  of  the  Upper  Cuspid. — This  may  or  may  not  be  a  dis- 
tinctive feature,  although  when  viewed  from  a  labial  aspect,  the  lateral 
flare  or  bulging  of  the  crown  gives  the  appearance  of  a  decided  con- 
striction between  the  crown  and  the  root;  but,  when  examined  from 
the  mesial  surface,  this  constricted  appearance  is  absent,  the  contour 
of  the  crown  passing  into  that  of  the  root,  with  the  cervical  line  alone 
marking  the  extent  of  each.  The  tooth  at  this  point  is  well  rounded 
anteriorly,  flattened  laterally,  and  again  rounded 
posteriorly,  the  latter  forming  the  segment  of  a 
smaller  circle  than  that  of  the  labial  surface. 

The  Root. — This  tooth  possesses  the  largest 
and  longest  root  of  any  of  the  teeth,  in  the  latter 
respect  usually  exceeding  the  central  incisor  by 
about  one-third,  and  the  lateral  incisor  by  one- 
fourth  or  more.  Like  the  base  of  the  crown,  it  is 
rounded  on  the  labial  and  lingual  surfaces,  and  is 
flattened  laterally,  this  form  usually  being  continued 
throughout  its  entire  length.  It  gradually  diminishes 
in  size  from  the  neck  to  the  apex,  and  in  its  entirety 
forms  a  perfect  cone.  On  the  mesial  and  distal 
sides  it  is  not  only  much  flattened,  but  is  frequently 
provided  with  a  longitudinal  depression,  which  is 
most  marked  near  the  center  of  its  length.  In 
some  instances  this  root  is  possessed  of  a  slight  distal 
curve,  which  may  be  gradual  from  the  base  to  the  apex,  or  it  may  exist 
in  a  more  positive  way  by  a  sudden  distal  curve  near  its  apical  extremity. 
Bilious  Type  (Fig.  72). — The  rounded  outlines  common  to  the 
cuspid  are  less  pronounced  in  this  type  than  in  any  other,  and  instead 
of  curves,  angles  are  present.  The  crown  is  above  the  average  size, 
length  predominating  over  breadth,  the  cusp  well  formed,  and  the 
angles  strong.  The  labial  surface  is  often  crossed  by  a  number  of  trans- 
verse ridges  near  the  cervical  portion,  the  labial  ridge  is  bold,  as  are  also 


Fig.  72. — Bilious  Type, 
Distal  Surface. 


UPPER   CUSPID  123 

the  labiomarginal  ridges.  The  mesial  and  distal  surfaces  possess  no 
distinguishing  features,  but  the  lingual,  like  the  labial,  shows  the  angular 
nature  of  the  crown  in  having  its  margins  and  ridges  squarely  set. 

A  cuspule  is  more  frequently  found  in  this  type  than  any  other, 
and  sometimes  reaches  down  to  a  point  corresponding  to  the  transverse 
center  of  the  crown.  The  neck  is  moderately  well  produced,  and  the 
cervical  line  decidedly  V-shaped  on  its  lateral  portions,  while  on  the  labial 
and  lingual  it  takes  the  form  of  a  broken  circle.  The  cutting-edges  are 
rather  heavy  and  square,  and  are  nearly  of  equal 
length.  In  this  temperament  the  cuspid  tooth 
often  partakes  of  the  form  described  in  connection 
with  the  lateral  incisor  of  the  same  type — i.  e.,  the 
absence  of  the  cutting-edge  and  one  or  both  angles, 
making  the  crown  a  perfect  cone. 

Nervous  Type  (Fig.  73). — The  crown  is  of 
much  greater  longitudinal  than  transverse  extent, 
the  outlines  oval  and  gracefully  formed,  and  the 
neck  is  much  constricted  from  mesial  to  distal, 
being  made  so  by  the  lateral  flare  of  the  body  of 
the  crown,  which  is  a  distinctive  feature  of  this 
type.  The  labial  ridge  is  well  formed  near  the 
summit  of  the  cusp,  but  usually  disappears  near 
the    center    of    the    surface.     The    labiomarginal       *ig.  73.— Nervous 

•  j  u  1     ,1  r  Type,    Labial    Surface, 

ridges    are    seldom    present,    and    the    surface    in 

general  is  convex  and  smooth.  The  mesial  and  distal  surfaces  show  a 
pronounced  convexity  near  the  angles,  and  often  a  slight  concavity 
between  this  point  and  the  cervical  line.  The  lingual  surface,  while 
generally  showing  all  the  descriptive  lines,  may  be  considered  smooth; 
it  is  convex  from  mesial  to  distal,  and  slightly  concave  in  the  direction 
of  the  long  axis  of  the  tooth.  The  cusp  is  long  and  penetrating,  the 
distal  cutting-edge  is  much  longer  than  the  mesial,  and  both  are  in- 
clined to  sharpness.  The  cervical  line  on  the  labial  and  lingual  surfaces 
is  deeply  arched,  frequently  giving  to  the  gingival  margin  a  receded 
appearance. 

Sanguineous  Type  (Fig.  74). — The  crown  in  this  type  abounds  in 
long  curves,  the  longitudinal  and  transverse  extents  are  nearly  equal, 
the  angles,  owing  to  their  circular  form,  are  barely  deserving  the  name 
while  the  cusp  and  cutting-edges  are  outlined  by  one  long,  oval  sweep. 
The  labial  surface  is  prominent  and  convex,  and  the  developmental 
grooves  are  fairly  well  shown.     The  mesial  and  distal  surfaces  show  a 


124 


ANATOMY 


moderate  general  convexity,  while  the  lingual  abounds  in  well-rounded 
ridges  and  borders.  The  constriction  forming  the  neck  of  the  tooth 
is  moderate.     The  cervical  line  is  in  the  form  of  perfectly  arched  curves, 

forming  on  the  labial  surface  the  segment  of 
a  circle  corresponding  to  the  circumference  of 
the  crown  of  the  tooth. 

Lymphatic  Type. — In  this  type  the 
crown  is  usually  greater  in  its  transverse  than 
in  its  longitudinal  measurement;  it  is  lacking 
in  graceful  outline,  and  may  best  be  described 
as  being  short,  thick,  and  heavy  set.  None 
of  the  surfaces  abound  in  descriptive  lines, 
although  transverse  ridges  are  sometimes 
present  on  the  labial  surface  near  the  cervix. 
Both  the  labial  and  lingual  surfaces  are  con- 
vex in  every  direction,  while  the  mesial  and 
distal  are  inclined  to  flatness.  The  cusp  is 
heavy  and  blunt,  and  the  cutting-edges,  which 
are  nearly  of  equal  length,  are  thick  and 
dull.  The  mesial  and  distal  angles  are  well 
produced.  The  cervical  line  is  almost  a 
direct  line  encircling  the  neck  of  the  tooth, 
the  segmental  form  on  the  labial  surface 
being  that  of  a  much  larger  circle  than  any  of  those  previously  described. 
The  neck  is  thick  and  heavy,  and  the  roots  generally  short. 


Fig.  74. — Sanguineous 
type,  Distal  Surface. 


UPPER    FIRST    BICUSPID  12  5 


UPPER  FIRST  BICUSPID. 


7th  year  8th  year  9th  year  10th  year  nth  year  12th  year 

Fig.  75. 

Calcification  Begins,  from  Four  Centers,  about  the  Fourth  Year. 
Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupted,  Tenth  to  Eleventh  Year. 
Average  Length  of  Crown,  .32. 

Average  Length  of  Root,  .48. 

Average  Length  over  All,  .80. 

This  tooth,  although  presenting  a  crown  of  vastly  different  contour, 
is  developed  by  a  process  almost  identical  with  that  of  the  incisors  and 
cuspids.  As  the  name  implies,  it  is  made  up  of  two  cusps,  one  forming 
the  buccal  and  the  other  the  lingual  half  of  the  crown.  Calcification 
in  the  buccal  cusp  is  from  three  centers  and  begins  about  the  fourth  year, 
the  central  lobe  first  receiving  the  lime  salts.  During  the  following 
year  the  two  lateral  lobes  begin  to  calcify,  soon  followed  by  a  union  of 
the  three,  thus  completing  the  margins  and  summit  of  the  cusp.  Unlike 
the  incisors,  but  corresponding  to  the  cuspid,  the  middle  lobe  is  much  the 
largest  of  the  three,  frequently  forcing  the  developmental  (buccal)  grooves 
well  toward  the  angles  of  the  crown.  The  development  of  the  lingual 
cusp  corresponds  to  the  development  of  the  cervical  ridge  on  the  incisors 
and  cuspids,  except  that  it  has  a  separate  center  of  calcification,  and  a 
cusp  almost  as  large  as  the  buccal  results.  Between  the  fifth  and  sixth 
year  union  between  the  two  cusps  takes  place,  the  line  of  confluence 
being  permanently  recorded  by  a  well-defined  groove,  which  traverses 
the  crown  from  mesial  to  distal.  This  groove,  although  forced  to  occupy 
a  different  position,  corresponds  to  the  lingual  groove  of  the  incisors 
and  cuspids.  After  the  union  of  the  cusps  the  process  of  calcification 
is  continued  into  the  body  of  the  crown,  and  by  the  seventh  year  it  is 


126 


ANATOMY 


Buccal  Triangular  Ridge 


more  than  half  completed.  The  eighth  year  usually  finds  the  crown 
fully  formed  and  the  base  of  the  root  or  roots  outlined.  As  this  tooth 
is  generally  provided  with  two  roots,  the  first  indication  of  bifurcation 
will  be  observed  between  the  eighth  and  ninth  years  by  a  filling-in  near 
the  center  of  the  mesial  and  distal  walls,  which  finally  become  united 
by  a  thin  septum  of  dentine  or  cementum.  After  this  period  the  roots 
calcify  separately,  and  by  the  middle  of  the  ninth  year  about  one-third 
of  their  length  is  established.  During  the  following  year,  or  at  about 
the  time  of  eruption,  the  development  of  the  roots  has  extended  to  about 
three-fourths  of  their  complete  length,  and  between  the  eleventh  and 

twelfth   years,  or  at   a    time   corres- 
ponding to  that  of  the  crown  assum- 
ing its  final  position  in  the  arch,  calci- 
:usP       fication  is  completed  (Fig.  75). 
roove  The  Crown  of  the  Upper  First 

pistai      Bicuspid    presents    for    examination 

iroove  r  r 

•istomar-  five  surfaces — buccal,  lingual,  mesial, 

ginal  '  °         '  ' 

Ridge  distal,  and  occlusal.  In  general,  the 
contour  of  the  crown  is  irregularly 
quadrilateral,  being  about  one-third 
greater  in  its  buccolingual  measure- 
ment than  from  mesial  to  distal.  It  is  somewhat  flattened  from  mesial 
to  distal,  but  rounded  on  its  buccal  and  lingual  surfaces. 

The  Occlusal  Surface  of  the  Crown  (Fig.  76). — The  contour 
of  the  crown  is  best  observed  by  a  view  of  this  surface,  which  may  be 
described  as  trapezoidal  or  irregularly  quadrilateral  in  form.  The 
four  margins  of  the  surface  are  those  which  represent  the  four  lateral 
surfaces — the  buccal,  lingual,  mesial,  and  distal — the  latter  two  being 
in  the  form  of  well-defined  ridges — the  mesio-  and  disto-mar ginal  ridges. 
The  buccal  margin  is  formed  by  the  mesial  and  distal  inclines  of  the 
buccal  cusp;  it  has  a  slight  buccal  convexity,  and  at  its  union  with  the 
proximate  surfaces  assists  in  forming  the  mesial  and  distal  angles  of  the 
crown.  The  distal  half  of  this  margin  is  usually  somewhat  longer  than 
the  mesial,  and  the  distal  angle  is  less  pronounced  than  the  mesial.  The 
lingual  margin  presents  a  much  greater  convexity  than  the  buccal,  but 
the  curve  formed  is  the  segment  of  a  much  smaller  circle.  As  in  the 
buccal  margin,  the  distal  half  of  this  margin  is  the  longest,  but  unlike 
the  former,  its  free  extremities  pass  into  the  mesial  and  distal  margins 
without  producing  angles.  The  mesio-  and  disto-marginal  ridges  are 
strong  folds  of  enamel  which  arise  from  the  mesial  and  distal  angles  and 


Lingual  Cusp 
Fig.  76. — Upper  First  Bicuspid, 
Occlusal  Surface. 


UPPER    FIRST    BICUSPID  127 

converge  slightly  as  they  pass  to  the  lingual,  where  they  are  gradually 
lost  in  the  lingual  margin. 

The  Cusps  (Fig.  76). — These  are  two  in  number,  and  are  named, 
in  accordance  with  their  location,  buccal  and  lingual.  The  buccal  cusp 
is  the  larger  and  longer  of  the  two.  From  the  summit  of  this  cusp  four 
ridges  descend — one  in  a  mesial  direction,  forming  the  mesial  cutting- 
edge  of  the  crown;  one  in  a  distal  direction,  forming  the  distal  cutting- 
edge;  one  to  the  buccal  surface,  the  buccal  ridge;  and  a  fourth,  the  buccal 
triangular  ridge,  descends  the  central  incline.  The  mesial  and  distal 
ridges  enter  into  the  formation  of  the  mesial  and  distal  angles  at  their 
extremities;  the  latter  is  slightly  longer  than  the  former,  and  both  are 
frequently  broken  near  the  center  by  the  grooves  of  development — the 
buccal  grooves.  The  buccal  ridge  may  be  well  developed  and  extend 
almost  to  the  cervical  line,  or  it  may  be  slight  and  disappear  near  the 
center  of  the  surface.  The  buccal  triangular  ridge  usually  ends  some- 
what abruptly  in  the  central  groove,  but  in  some  instances  it  is  con- 
tinued and  joins  a  similar  ridge  from  the  lingual  cusp,  this  union  form- 
ing the  transverse  ridge.  The  triangular  ridge  often  bifurcates  near 
the  center  "of"  its  incline,  and  is  continued  in  two  distinct  but  smaller 
ridges.  The  lingual  cusp  is  much  less  angular  than  the  buccal;  the  apex 
is  usually  rounded,  while  the  descending  ridges  are  generally  three  in 
number  instead  of  four.  The  mesial  and  distal  ridges  are  nearly  of  the 
same  length,  and  pass  without  interruption  into  the  mesio-  and  disto- 
marginal  ridges.  The  triangular  ridge  is  less  .clearly  defined  than  its 
fellow  of  the  buccal  cusp,  and  it  is  not  unusual  for  it  to  be  entirely 
wanting.  The  lingual  aspect  of  the  cusp  is  smooth  and  rounded, 
presenting  nothing  in  the  form  of  a  ridge  in  correspondence  with  the 
buccal  ridge  of  the  buccal  cusp.  Like  the  incisors  and  cuspids,  the 
summits  of  these  cusps  are  usually  in  a  direct  line  with  the  long  axis 
of  the  tooth. 

The  developmental  grooves,  all  of  which  are  observed  upon  the  occlusal 
surface,  are  the  central,  mesial,  distal,  two  triangular,  and  two  buccal. 
The  central  groove  is  the  most  marked,  is  deeply  sulcate,  and  extends 
through  the  center  of  the  surface  from  mesial  to  distal,  ending  just 
within  the  two  marginal  ridges  in  two  irregularly  formed  depressions 
or  pits — the  mesial  and  distal  pits.  This  groove  marks  the  line  of  union 
between  the  buccal  and  lingual  lobes.  The  mesial  and  distal  grooves 
are  not  always  well  denned,  but  may  usually  be  observed  as  fine  lines 
passing  over  the  central  portion  of  the  mesio-  and  disto-marginal  ridges. 
The  mesio-  and  disto-triangular  grooves  becin  in  the  mesial  and  distal 


128 


ANATOMY 


HFt^^^H 

Buccal 

BF     i    ^H 

Ridge 

^K                       ^^^J 

[Mesial 

Br                .     ^H 

Angle 

KwBK 

Distal 
Angle 


Distal  cut- 
ting-edge 


Buccal  Grooves 
Fig.  77. — Upper  First  Bicuspid, 
Buccal  Surface. 


pits,  and  pass  in  the  direction  of  the  mesial  and  distal  angles,  where  they 
are  either  lost,  or  may  be  traced  as  slight  depressions  passing  over  the 
buccal  ridges  near  the  angles,  and  they  may  further  continue  over  the 

buccal  surface  in  the  direction  of  the 
root.  These  two  grooves,  together 
with  the  mesial  and  distal  above  re- 
ferred to,  form  the  outlines  of  the 
mesiobuccal  and  distobuccal  develop- 
mental lobes.  The  buccal  grooves 
will  be  described  in  connection  with 
the  buccal  surface.  Supplemental 
grooves  are  seldom  found  in  connec- 
tion with  the  buccal  half  of  the  oc- 
clusal surface,  but  are  occasionally 
present  on  the  central  incline  of  the 
lingual  cusp. 

The  Buccal  Surface  of  the  Crown 
(Fig.  77).- — In  many  respects  this  surface 
resembles  the  corresponding  or  labial  surface  of  the  cuspid.  It  is 
bounded  by  four  margins — occlusal,  mesial,  distal,  and  cervical.  The 
occlusal  half  of  the  surface  is  formed  of  the  buccal  cusp,  and  is  cone- 
shaped,  while  the  cervical  half  is  irregu- 
larly quadrilateral  in  form.  The  extent 
of  surface  from  the  cervical  line  to  the 
point  of  the  cusp  is  usually  about  one- 
third  greater  than  the  greatest  mesio- 
distal  diameter,  which  is  represented  by 
a  line  drawn  from  the  mesial  to  the 
distal  angle.  The  form  shown  is  that 
of  a  general  convexity,  the  summit  of 
which  is  surmounted  by  a  longitudinal 
ridge — the  buccal  ridge.  This  ridge, 
which  is  formed  from  the  central  de- 
velopmental lobe,  is  most  pronounced 
near  the  occlusal  margin,  and  gradually 
disappears  near  the  center  of  the  surface. 
Upon  either  side  of  the  buccal  ridge  are 

two  grooves — the  buccal  grooves — which  denote  the  line  of  union  between 
the  central  and  the  two  lateral  lobes,  and  beyond  these  are  the  angles  of 
the  crown.     The  buccal  ridge  springs  from  the  buccal  cusp,  the  summit  of 


Summit  of  Lingual  Cusp 

Fig.    78.- — Upper  First   Bicuspid, 

Lingual  Surface. 


UPPER    FIRST    BICUSPID  1 29 

which  is  generally  in  a  direct  line  with  the  long  axis  of  the  tooth;  when 
there  is  a  deviation  from  this  the  summit  is  usually  thrown  a  little  to 
the  mesial,  resulting  in  a  reduction  of  the  length  of  the  mesial  cutting-edge. 

As  previously  stated,  the  greatest  mesiodistal  diameter  of  the  surface 
is  on  a  line  with  the  angles  of  the  crown;  this  measurement  is  much  reduced 
at  the  cervical  line,  so  that  the  point  of  contact  with  adjoining  teeth  is 
thrown  near  the  occlusal  margins  of  the  crown.  This  variation  in  the 
transverse  measurement  also  results  in  what  is  commonly  referred  to  as 
the  "bell  shape"  of  the  crown.  The  cervical  margin  of  the  surface  is 
fairly  well  arched,  but  seldom  to  such  a  degree  is  the  corresponding 
margin  upon  the  incisors  and  cuspids. 

The  Lingual  Surface  of  the  Crown   (Fig.   78). — This   surface  is 
smooth  and  decidedly  convex,  the  absence  of  strongly  developed  grooves 
and  ridges  contributing  to  the  former  fact.     Like  the  buccal  surface, 
its    greatest   transverse    measure- 
ment is  at  the  base  of  the  cusp  in 
which  it  terminates.     The  extent 
of  the  surface  is  about  one-third 
less  than  that  of  the  buccal,  and 
its  occluding  and  cervical  margins 
are  alone  well  defined,  the  mesio- 
distal convexity  passing  so  gradu- 
ally into  these  respective  surfaces 
that  a  positive  line  of  distinction 
can   scarcely  be   recognized.     In 
passing  from  the  cervical  line  to 
the  occlusal  margin,  the  surface  is         FlG  79._Upper  First  Bicuspid,  Mesial 
rapidly  carried  toward  the  center  Surface. 

of  the  crown.  The  cervical  margin  of  this  surface  is  usually  in  the  form 
of  a  direct  line  encircling  the  neck,  but  occasionally  presents  a  slight 
concavity  in  the  direction  of  the  root. 

The  Mesial  Surface  of  the  Crown  (Fig.  79). — This  surface  of  the 
crown  has  three  of  its  borders  well  defined;  these  are  the  buccal,  the 
occlusal,  and  the  cervical,  the  remaining  or  lingual  margin  passing  so 
gradually  into  the  lingual  surface  that  no  positive  line  of  demarcation 
can  be  given.  The  buccal  margin  extends  from  the  mesial  angle  to  the 
cervical  line,  and  invariably  presents  a  slight  buccal  inclination,  thus 
increasing  the  width  of  the  surface  on  its  cervical  portion.  The  occlusal 
margin  is  formed  by  the  mesiomarginal  ridge,  and  by  a  portion  of  the 
ridge  descending  from  the  lingual  cusp.  It  is  irregularly  V-shaped, 
9 


130 


ANATOMY 


Buccal 
Ridge 


and  in  many  instances  is  broken  in  the  center  by  the  mesial  groove.  The 
cervical  margin  differs  from  those  of  the  incisors  and  cuspids,  nearly 
always  being  in  the  form  of  a  straight  line  from  buccal  to  lingual.  The 
surface  in  general  is  flattened,  but  shows  a  slight  general  convexity  near 
the  occlusal  margin,  and  frequently  a  slight  concavity  immediately  below 
the  cervical  line,  this  form  placing  the  point  of  contact  with  the  distal 
surface  of  the  cuspid  near  the  occlusal  margin.  The  surface  is  occa- 
sionally divided  into  a  buccal  and  a  lingual  portion  by  the  mesial  groove, 
which  may  extend  to  the  cervical  line,  but  which  generally  disappears 

near  the  center  of  the  surface. 
The  buccal  half  of  the  surface 
which  is  formed  from  the  mesial 
developmental  lobe  is  inclined  to 
Buccal  Root  angularity,  while  the  lingual  half 
is  decidedly  rounded,  particularly 
in  the  direction  of  the  occlusal 
margin. 

The  Distal  Surface  of  the 
Crown    (Fig.    80). — In    general, 
this  surface  resembles  the  mesial, 
being  flattened  and  bounded  by 
three  more  or  less  distinct  mar- 
gins.    The     slight     buccolingual 
convexity  is  not  confined  to  the 
occlusal   portion   of  the  surface, 
but  is  inclined   to  extend  to  the 
cervical  margin,  in  this  particular  being   at  variance  with  the  mesial 
surface.     This  surface  passes  into  the  lingual  by  a  much  longer  curve 
than  that  shown  on  the  mesial  surface. 

The  Angles  of  the  Crown. — These  are  two  in  number  and,  as  in 
the  teeth  previously  described,  are  named,  according  to  their  location, 
mesial  and  distal.  The  mesial  angle  is  formed  by  the  union  of  the  mesio- 
marginal  ridge  and  mesial  cutting-edge.  It  is  primarily  the  product  of 
the  mesial  developmental  lobe,  and  is  usually  well  produced.  The 
distal  angle,  which  is  formed  in  a  like  manner,  is  inclined  to  be  more 
rounded. 

The  Neck  of  the  Tooth. — In  most  typal  forms  the  neck  of  the 
upper  first  bicuspid  is  well  defined,  particularly  upon  the  mesial  and 
distal  surfaces.  Viewing  the  tooth  from  a  buccal  aspect,  the  neck  is 
a  distinctive  feature,  but  when  studied  from  the  mesial  or  distal  sides, 


Triangular  Ridge, 
Occlusal  Surface 

Fig.  80. — Upper  First  Bicuspid,  Distal 
Surface. 


UPPER    FIRST    BISCUPID  131 

the  constriction  is  scarcely  observed,  this  being  particularly  the  case  if 
the  tooth  has  but  a  single  root.  In  general,  the  neck  partakes  of  the 
contour  of  the  crown,  being  convex  on  the  buccal  and  lingual,  and  flat- 
tened and  frequently  slightly  concave  on  the  mesial  and  distal. 

The  Roots  of  the  Upper  First  Bicuspid. — This  tooth  is  usually 
developed  with  two  roots,  sometimes  with  only  one,  and  in  rare  instances 
it  may  have  three.  When  two  roots  are  present,  one  is  above  the  buccal 
and  the  other  above  the  lingual  half  of  the  crown,  and  are  named,  accord- 


Fig.  81. — Types  of  Bicuspids. 

ing  to  their  location,  as  buccal  and  lingual.  In  general  form  the  tfwo 
roots  are  quite  similar,  but  the  buccal  is  usually  a  trifle  longer  than  the 
lingual.  They  taper  off  to  a  slender  apex,  and  are  inclined  to  curve  in 
various  directions  near  their  extremities.  The  point  of  bifurcation  is 
frequently  some  distance  above  the  neck,  so  that  the  tooth  may  be  said 
to  possess  a  single  root  with  two  branches.  Below  the  bifurcation  the 
root  assumes  the  form  of  the  neck  or  cervical  portion  of  the  crown,  but 
as  the  bifurcation  is  approached,  the  mesial  and  distal  sides  present  a 
longitudinal  groove,  which  gradually  increases  in  depth  until  the  single 
root  becomes  separated.  The  curves  of  the  root-branches  above  referred 
to  are,  in  the  buccal  branch,  first  to  the  buccal  and  then  to  the  lingual; 
while  the  lingual  branch  first  shows  a  slight  lingual  inclination  immedi- 
ately above  the  point  of  separation,  followed  by  a  gentle  buccal  curve 
as  the  apical  end  is  reached.     In  some  cases  the  bifurcation  begins 


132 


ANATOMY 


immediately  above  the  neck  of  the  tooth;  in  others  it  may  occur  in  the 
apical  third;  while  a  third  class  is  represented  by  the  two  roots  being 
united  throughout  their  entire  length  by  a  thin  septum  of  dentin  and 
cementum,  or,  as  occasionally  happens,  by  a  layer  of  cementum  alone. 
In  this  latter  instance  each  root  is  provided  with  a  distinct  canal.  When 
the  tooth  has  but  a  single  root,  it  is  much  flattened  from  mesial  to  distal, 
the  flatness  being  slightly  broken  by  an  inclination  to  convexity.  The 
four  surfaces  usually  converge  toward  the  apical  end,  which  is  oblong  from 
buccal  to  lingual,  and  generally  provided  with  a  slight  distal  curve.     The 

presence  of  three  roots  is  so  rare  that 
the  condition  might  be  classed  as  a 
malformation;  but  when  they  do  exist, 
two  are  usually  attached  to  the  buccal 
and  one  to  the  lingual  half  of  the 
crown,  with  the  point  of  separation 
near  the  neck  of  the  tooth. 

Bilious  Type  (Fig.  82).— The 
upper  first  bicuspid  of  this  tempera- 
mental type  is  marked  by  a  crown  of 
moderate  length,  the  neck  well  pro- 
nounced, and  the  cusps  and  angles 
marked  by  angular  outlines.  The 
buccal  ridge  is  strongly  defined,  and 
the  buccal  grooves,  which  extend  well 
up  on  the  buccal  surface,  cross  the  mesial  and  distal  cutting-edges,  sepa- 
rating them  into  two  distinct  parts.  The  cusps  are  long  and  penetrating, 
and  are  nearly  of  equal  length,  assisting  to  form  the  firm  and  well-locked 
occlusion  common  to  this  type.  The  mesial  and  distal  surfaces  are  nearly 
parallel  with  each  other;  they  are  seldom  convex,  so  that  the  approximating 
teeth  are  in  contact  over  an  extent  of  surface  rather  than  a  single  point. 
The  cervical  line  is  but  little  curved. 

Nervous  Type  (Fig.  83). — In  this  temperament  the  bell-shaped 
crown  is  strongly  observed,  the  crown  being  long  and  much  con- 
stricted at  its  neck.  The  extreme  length  of  the  buccal  cusp,  and  the 
marked  cervical  constriction,  produce  an  appearance  in  the  buccal 
surface  resembling  the  labial  surface  of  the  cuspid  tooth.  The  develop- 
mental grooves  are  finely  outlined,  and  the  cusps  long  and  penetrating, 
usually  being  more  pronounced  than  in  any  other  class.  The  cutting- 
edges  are  sharp  and  inclined  to  angularity;  the  mesial  and  distal  surfaces 


Fig.  82. — Bilious  Type,  Distal 
Surface. 


UPPER    FIRST    BICUSPID 


133 


are  convex  near  their  occlusal  margins,  but  near  the  cervical  line  a 
pronounced  concavity  is  observed,  which  is  continued  upon  the  correspond- 
ing root-surfaces.  This  formation  forces  the  point  of  contact  with 
adjoining  teeth  well  toward  the  occlusal  surface,  and  results  in  an  exten- 
sive V-shaped  interproximate  space.  The  cervical  line  is  sharply  and 
gracefully  formed,  the  curvature  being  well  arched. 

Sanguineous  Type. — The  typical  upper  first  bicuspid  of  this  class 
is  provided  with  a  crown  well  proportioned,  its  length  being  somewhat 
greater  than  its  breadth,  but  about  equal  to  its  buccolingual  measure- 
ment. The  buccal  surface  is  seldom  broken  by  the 
buccal  grooves,  and  is  strongly  convex  in  every  direc- 
tion. The  lingual  surface  is  much  more  rounded 
than  the  same  surface  of  other  types.  The  mesial 
and  distal  surfaces  are  usually  smoothly  convex,  with 
an  occasional  slight  concavity  immediately  below  the 
cervical  line.  Upon  the  occlusal  surface  the  grooves 
are  rounded  and  obscure,  rather  than  sharp  and  well 
defined,  and  the  cusps,  much  less  pronounced  than  in 
either  of  the  types  previously  described,  are  rounded 
and  smooth;  this  latter  fact  is  particularly  true  of  the 
lingual  cusp,  which  is  usually  much  smaller  than  the 
buccal.  The  cutting-edges  of  the  buccal  cusp  are 
scarcely  deserving  of  the  name,  being  broad  and 
rounded  throughout.  The  form  of  the  mesial  and 
distal  surfaces  above  described  provides  for  a  point  of  contact  near  the 
center  of  each  surface,  leaving  a  slight  interproximate  space  both  above 
and  below  this  point. 

Lymphatic  Type. — An  examination  of  the  upper  first  bicuspid  of 
the  lymphatic  type  results  in  finding  a  tooth  vastly  different  from  any 
of  those  previously  described.  The  length  of  the  crown  from  the  cervical 
line  to  the  point  of  the  cusp  is  less  than  either  the  mesiodistal  or  bucco- 
lingual measurements.  In  general  appearance  it  is  lacking  in  symmetry, 
or  poorly  proportioned.  The  buccal  surface  presents  a  gradual  convexity 
from  mesial  to  distal,  and  seldom  has  the  buccal  ridge  well  developed. 
The  lingual  surface  is  smoothly  convex  and  passes  off  into  the  lingual 
root  without  the  interposition  of  a  decided  neck.  The  mesial  and  distal 
surfaces  are  flattened  and  sparingly  convex,  and  are  nearly  parallel  with 
each  other,  so  that  the  contact  with  adjoining  teeth  is  inclined  to  be 
distributed  over  the  entire  surface,  leaving  little  or  no  interproximate 
space.     The  cusps  are  short,  flat,  and  rounded,  and  the  occlusal  surface 


Fig.    83. — Nerv- 
ous    Type,     Buccal 

Surface. 


134  ANATOMY 

much  flattened  in  general,  corresponding  with  the  nature  of  the  occlusion, 
which  is  loose  and  wandering.  The  developmental  grooves  and  ridges 
are  fairly  well  shown,  while  the  cutting-edges  and  angles  of  the  crown 
are  smooth  and  rounded.  The  neck  is  less  pronounced  in  this  type  than 
in  any  other,  the  curvature  of  the  cervical  line  is  very  slight,  the  root  is 
short  and  heavy  set,  frequently  passing  well  up  toward  the  apex  before 
bifurcating. 


UPPER    SECOND    BICUSPID  135 


UPPER  SECOND  BICUSPID. 


7th  year  8th  year  9th  year  10th  year  nth  year  12th  year 

FIG.  84. 

Calcification  Begins,  from  Four  Centers,  about  the  Fifth  Year. 
Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupted,  Eleventh  to  Twelfth   Year. 
Average  Length  of  Crown,  .29. 

Average  Length  of  Root,  .55. 

Average  Length  over  All,  .84. 

The  process  of  development  in  this  tooth  is  identical  with  that  of 
the  first  bicuspid,  calcification  in  the  buccal  half  of  the  crown  taking 
place  in  one  central  and  two  lateral  lobes,  while  the  lingual  half  is  developed 
from  a  single  center.  The  calcifying  process  is  about  one  year  later 
than  that  in  the  first  bicuspid,  the  summit  of  the  buccal  cusp  receiving 
its  lime  salts  about  the  beginning  of  the  fifth  year.  During  the  following 
six  months  calcification  begins  in  the  lateral  lobes,  and  also  in  the  lingual 
lobe.  By  the  sixth  year  the  occlusal  surface  and  a  portion  of  the  crown 
are  completed  by  a  union  of  the  various  lobes,  and  at  seven  years  the 
crown  is  calcified  for  more  than  two-thirds  of  its  completed  length. 
Between  the  eighth  and  ninth  year  the  contour  of  the  crown  is  established, 
and  the  neck  of  the  tooth  and  outline  of  the  root-base  formed.  At  the 
tenth  year  about  one-third  of  the  root-length  is  formed,  and  during  the 
following  year  about  1/8  of  an  inch  is  added  to  it.  By  the  eleventh  or 
twelfth  year  calcification  is  completed  (Fig.  84). 

This  tooth  so  closely  resembles  the  first  bicuspid  that  a  description 
in  detail  will  be  unnecessary;  there  are,  however,  a  few  minor  points 
which  are  at  variance  and  must  be  described  in  order  to  distinguish  one 
from  the  other.  In  general,  the  tooth  is  a  trifle  smaller  than  the  first 
bicuspid,  the  cusps  are  somewhat  shorter,  and  the  various  ridges  less 


136  ANATOMY 

distinct.  A  distinguishing  feature  of  the  occlusal  surface  is  found  in  the 
diminished  length  of  the  central  groove  (Fig.  88).  This  groove,  as 
observed  in  the  first  bicuspid,  extends  from  mesial  to  distal  for  fully 
three-fourths  of  the  entire  width  of  the  surface;  but  in  the  second  bicuspid 
it  is  diminished  by  one-third,  being  thus  reduced  by  the  broadened  mar- 
ginal ridges,  which  force  the  mesial  and 
distal  pits  well  toward  the  center.  It  is  not 
uncommon  to  find  the  triangular  grooves 
joining  the  central  groove  directly  in  the 
Trianguiarcenter  of  the  surface,  forming  a  central  pit 

Grooves 

from  which  may  radiate  numerous  small 
supplemental     grooves     and    ridges.     The 
summits  of    both  the  buccal   and   lingual 
Fig.  85  — Second  Upper  Bicuspid    cusps  are  nearer  to  the  mesial  than  to  the 

Occlusal  Surface.  , .        .  r  ,  .  .  ,       ,  ,       . 

distal  surface,  thus  increasing  the  length  of 
the  ridges  which  descend  from  them  in  a  distal  direction,  and  decreasing 
those  which  pass  to  the  mesial.  The  buccal  surface  presents  a  greater 
convexity  than  that  of  the  first  bicuspid;  the  buccal  grooves  are  usually 
shallow  depressions  and  are  frequently  entirely  wanting,  thus  giving  the 
buccal  ridge  the  appearance  of  extending  its  margins  to  the  angles  of 
the  crown.  Unlike  the  first  bicuspid, 
the  mesiodistal  diameter  of  the  mesial 
surface  is  but  little  more  than  the 
same  measurements  on  the  lingual 
surface.  The  neck  of  the  tooth  is 
not  quite  so  pronounced  as  that  of 
the  first  bicuspid,  thus  giving  less  of 
the  bell  shape  to  the  crown.  One 
very  important  difference  between 
this  tooth  and  the  first  bicuspid  is  in 
the  root-formation.     We  have  seen  that 

the    first     bicuspid     is     generally     pro-  MIMI  Klll):i        ^^  ^Lingual 

vided    with    two    roots,    while    in    the 
second  bicuspid  a  single  root  is  usually 

nresent  T  ikp  the  first  himeniH  FlG-  86-— uPPer  Second  Bicuspid,  Me- 
presenr.       ±,lKe        tne        nrst         bicuspid,         s;al  Surface.     Most  common  form. 

there  are  exceptions  to  this,   the  tooth 

sometimes  being  provided  with  two,  and  in  rare  instances  with  three, 
roots.  When  the  single  root  is  present,  it  partakes  of  the  form  of 
the  crown  at  its  base,  being  well  rounded  on  the  buccal  and  lingual 
portions  and  much  flattened  on  the  mesial  and  distal.     The  mesiodistal 


UPPER    SECOND    BICUSPID 


137 


diameter  of  the  rcot  at  its  base  is  only  about  one-third  that  of  the  bucco- 
lingual  measurement,  this  proportionate  size  continuing  throughout 
its  entire  length.  In  passing  from  the  base  to  the  apex  the  root  is  gradu- 
ally diminished  in  size,  finally  ending  somewhat 
abruptly  in  an  oblong  extremity.  In  some  in- 
stances the  apical  end  is  round  and  pointed, 
resembling  the  apex  of  the  incisors  and  cuspids, 
but  when  thus  formed  the  root  is  usually  curved 
near  its  apical  third  and  somewhat  extended  in 
length.  The  mesial  and  distal  surfaces  are  pro- 
vided with  a  well-defined  longitudinal  concavity, 
extending  from  the  cervical  margin  to  the  apex  and 
dividing  the  root  into  a  buccal  and  a  lingual  por- 
tion. This  depresson  is  often  so  decided  that  the 
contour  of  the  single  root  is  almost  lost,  and  in  its 
place  the  appearance  is  that  of  two  roots  similar  to 
those  described  in  the  first  bicuspid.  The  length 
of  the  root  is  usually  a  little  greater  than  that  of 
the  first  bicuspid,  but  the  crown  being  a  trifle 
shorter,  results  in  producing  a  tooth  the  entire  length  of  which  is  about 
equal  to  that  of  the  first  bicuspid. 


Fig.  87. — Young  Up, 
per  Second  Bicuspid  - 
Linguaf  Surface. 


ANATOMY 


UPPER  FIRST  MOLAR. 


2d  year 


3d  year        5th  year 


6th  year 

Fig.  8i 


7th  year 


10th  year        nth  year 


Calcification  Begins,  from  Four  Centers,  about  One  Month  Before  Birth. 
Calcification  Completed,  Ninth  to  Tenth  Year. 
Erupted,  Sixth  to  Seventh  Year. 
Average  Length  of  Crown,  .30. 

Average  Length  of  Root,  .51. 

Average  Length  over  All,  .81. 

This  tooth  being  the  first  of  the  permanent  organs  to  erupt,  pre- 
cedes all  others  in  the  process  of  calcification,  beginning  to  receive  its 
lime  salts  as  early  as  the  eighth  fetal  month.  The  form  of  the  crown 
being  so  entirely  different  from  those  previously  described,  embodies 
a  developmental  process  which  is  also  different,  four  distinct  lobes  being 
present,  one  for  each  cusp,  these  making  their  appearance  during  the 
first  year  after  birth,  closely  followed  by  a  completion  of  the  occlusal 
surface  by  the  union  of  the  free  calcifying  margins,  these  lines  of  union 
being  finally  represented  by  the  developmental  grooves  of  the  occlusal 
surface.  After  the  completion  of  this  surface,  calcification  proceeds  in 
the  direction  of  the  base  of  the  crown,  and  at  the  beginning  of  the  third 
year  about  two-thirds  of  the  crown  is  formed.  During  the  fifth  year 
the  contour  of  the  crown  is  completed,  and  at  the  beginning  of  the  eruptive 
period,  or  about  the  sixth  year,  the  developmental  process  has  extended 
to  the  base  of  the  roots,  and  an  effort  at  trifurcation  begun.  At  seven 
years  the  three  roots  with  which  the  tooth  is  provided  are  branching  out, 
each  into  its  own  socket,  subsequent  development  in  each  being  con- 
tinued as  a  separate  and  distinct  process.  The  tenth  year  more  than 
half  completes  the  calcifying  process  in  the  roots,  and  at  the  beginning 
of  the  eleventh  year  the  root  apices  are  formed.  Like  the  first  bicuspid, 
the  crown  of  this  tooth  presents  for  examination  five  surfaces — occlusal, 
buccal,  lingual,  mesial,  and  distal.     In  general  contour  it  is  irregularly 


UPPER    FIRST    MOLAR  1 39 

quadrilateral,  with  the  angles  of  the  crown  more  or  less  rounded,  two 
of  its  sides  convex  and  two  flattened  or  slightly  concave.  The  length 
of  the  crown  from  the  cervical  line  to  the  summits  of  the  cusps  is  about 
equal  to,  or  slightly  less  than,  its  mesiodistal  diameter,  while  the  bucco- 
lingual  measurement  is  usually  a  trifle  greater  than  the  mesiodistal. 
The  Occlusal  Surface  of  the  Crown  (Fig.  89). — The  coronal 
outline  of  this  tooth  is  best  studied  when  looking  directly  upon  this 
surface,  which  shows  the  two  convex  sides  above  referred  to,  represented 

Mesiobuccal  Cusp 


Buccal  Groove 


Central  Fossa 

Oblique  Ridge 

Mesiolingual  Cusp 
The  Fifth  Cusp 


Distolingual      Distolingual 
Groove  Cusp 

Fig.  89.  Upper  First  Molar,  Occlusal  Surface. 

by  the  buccal  and  lingual  margins,  with  the  mesial  and  distal  margins 
more  or  less  flattened.  The  surface  is  bounded  by  these  four  margins, 
which  are  nearly  of  equal  length,  the  angles  formed  by  their  union  being 
more  or  less  rounded,  two  of  which,  the  mesiobuccal  and  the  distolingual, 
are  acute  angles,  while  the  mesiolingual  and  distobuccal  are  obtuse  angles. 
The  surface  is  divided  into  four  developmental  portions — the  mesiobuccal, 
distobuccal,  mesiolingual,  and  distolingual.  Each  one  of  these  parts 
is  surmounted  by  a  well-defined  point  or  cusp,  which  likewise  is  named 
in  accordance  with  its  location.  These  various  parts  are  separated  from 
one  another  by  four  developmental  grooves — the  mesial,  the  buccal, 
the  distal,  and  the  distolingual.  In  the  center  of  the  triangle  formed 
by  the  central  incline  of  the  mesiobuccal,  distobuccal,  and  mesiolingual 
cusps  is  a  deep  depression — the  central  fossa — while  near  the  distal 
margin  is  a  somewhat  similar  depression — the  distal  fossa.  Traversing 
the  surface  in  various  directions  are  a  number  of  ridges  and  supplemental 
grooves,  each  of  which  will  be  described  in  turn. 

The  Marginal  Ridges  of  the  Occlusal  Surface. — These  are  four  in 
number — the  mesial,  distal,  buccal,  and  lingual.  The  mesiomarginal 
ridge  is  a  well-pronounced  elevation  of  enamel  which  passes  from  the 
mesiobuccal  angle  to  the  mesiolingual  angle.     It  is  slightly  concave  in 


140  ANATOMY 

the  direction  of  the  root,  and  is  broken  near  the  center  of  its  concavity 
by  the  mesial  groove,  upon  either  side  of  which  are  frequently  found  one 
or  two  small  points  or  tubercles,  which  are  formed  either  by  a  division 
of  the  mesial  developmental  groove,  or  by  one  or  more  supplemental 
grooves.  These  grooves  pass  over  the  ridge  and  are  continued  for  a  short 
distance  on  the  mesial  surface.  Descending  from  the  mesiobuccai  cusp, 
the  ridge  passes  in  a  lingual  direction  to  meet  the  mesiolingual  cusp,  and 
in  so  doing  has  a  slight  distal  inclination  until  the  mesial  groove  is  reached, 
after  passing  which  it  makes  a  sweeping  distal  curve  and  is  lost  in  the 
lingual  margin.  This  ridge  marks  the  line  of  junction  between  the 
occlusal  surface  and  the  mesial  surface.  The  distomarginal  ridge  in 
some  respects  resembles  the  mesial  just  described,  being  concave  and 
ascending  in  a  buccal  and  lingual  direction,  with  a  somewhat  rounded 
outline,  to  the  summits  of  the  distobuccal  and  distolingual  cusps.  The 
depth  of  the  concavity  is  usually  greater  than  that  of  the  mesial  margin, 
and  is  frequently  crossed  near  the  center  by  the  distolingual  groove, 
frequently  so  marked  as  to  produce  a  V-shape  to  the  center  of  the  margin. 
There  are  occasionally  found  upon  either  side  of  this  central  groove  one 
or  more  small  tubercles,  corresponding  to  those  of  the  mesial  ridge,  but 
they  are  less  frequent  and  less  pronounced.  This  ridge  forms  the  line 
of  demarcation  between  the  occlusal  surface  and  the  distal  surface.  The 
buccomarginal  ridge  begins  at  the  mesiobuccai  angle,  and  gradually 
ascends  to  the  summit  of  the  mesiobuccai  cusp,  from  which  it  afterward 
descends  in  a  distal  direction  to  the  buccal  groove;  continuing,  it  again 
ascends  the  distobuccal  cusp,  after  descending  from  which  it  ends  in  the 
distobuccal  angle.  The  nature  of  this  ridge  is  a  series  of  cutting-edges, 
giving  to  the  cusps  their  angular  nature.  Besides  the  buccal  groove, 
which  makes  a  decided  break  in  the  center  of  its  course,  the  ridge  is 
frequently  crossed  by  numerous  small  supplemental  grooves  occurring 
in  various  locations  and  forming  a  series  of  minute  tubercles;  this  latter 
condition  is  most  frequently  present  in  young  teeth,  and  is  soon  obliterated 
by  wear.  The  course  of  this  ridge  is  not  that  of  a  direct  line  from  mesial 
to  distal,  but  in  its  ascent  of  the  mesiobuccai  cusp  it  is  inclined  to  the 
buccal;  in  its  descent  it  presents  a  corresponding  return  to  the  lingual, 
and  the  same  variations  are  observed  in  passing  over  the  distal  cusp. 
The  linguomarginal  ridge  begins  at  the  mesiolingual  angle  of  the  crown 
and  passes  distally  to  the  distolingual  angle,  differing  from  the  three 
previously  described  by  being  heavy  and  rounded  in  its  nature,  more 
irregular  in  outline,  and  divided  nearest  to  its  distal  extremity  instead 
of  in  the  center  of  its  length.     From  the  point  of  beginning  it  makes  a 


UPPER    FIRST    MOLAR  141 

curved  ascent  to  the  summit  of  the  mesiolingual  cusp;  descending  from 
this  in  a  distobuccal  direction,  it  divides,  one  portion  passing  to  join  the 
triangular  ridge  of  the  distobuccal  cusp,  the  two  uniting  to  form  the 
oblique  ridge,  the  other  portion  continuing  in  the  direction  of  the  disto- 
lingual  cusp,  before  reaching  the  base  of  which  it  is  broken  by  the  disto- 
lingual groove.  From  this  groove  the  ridge  makes  a  sudden  and  direct 
ascent  to  the  summit  of  the  distolingual  cusp,  after  passing  which  it 
gradually  descends  in  a  long  curve  to  join  the  distomarginal  ridge.  Like 
the  buccal  'ridge,  it  is  frequently  crossed  by  numerous  supplemental 
grooves.  The  ridge  forms  the  lingual  margin  of  the  occlusal  surface 
and  gives  to  the  cusps  their  angularity. 

The  Cusps  (Fig.  89). — These  are  four  in  number — the  mesiobuccal, 
distobuccal,  mesiolingual,  and  distolingual. 

The  Mesiobuccal  Cusp  (Fig.  89). — In  extent  of  surface  this  is  usually 
the  largest  cusp,  although  it  is  sometimes  exceeded  by  the  mesiolingual. 
From  the  summit  of  the  cusp  three  ridges  descend — the  buccal  ridge  to 
the  buccal  surface,  the  buccomarginal  ridge  making  a  double  descent, 
and  the  mesiobuccal  triangular  ridge,  the  latter  descending  the  central 
incline  and  ending  in  the  central  fossa.  The  mesial  base  of  the  cusp  is 
frequently  crossed  by  one  or  more  supplemental  grooves,  which  begin  at 
the  mesial  margin  and  pass  in  the  direction  of  the  central  fossa.  The 
central  slope  of  the  cusp  contributes  to  the  formation  of  the  central  fossa, 
its  extent  in  this  direction  being  controlled  by  the  mesial  and  buccal 
grooves,  which  together  form  the  mesiobuccal  triangular  groove. 

The  Distobuccal  Cusp  (Fig.  89). — This  cusp  is  frequently  the  smallest 
in  extent  of  surface,  but  is  usually  longer  and  more  pointed  than  the  others. 
Like  the  mesiobuccal  cusp,  three  ridges  descend  from  it — the  buccal 
ridge  to  the  buccal  surface  two  which  spring  from  the  buccomarginal 
ridge,  and  the  distobuccal  triangular  ridge,  which  descends  obliquely 
toward  the  distal  center  of  the  surface  and  joins  a  similar  ridge  (pre- 
viously described)  from  the  mesiolingual  cusp,  the  two  forming  the 
oblique  ridge.  The  mesial  portion  of  the  base  of  this  cusp  assists  in 
forming  the  central  fossa,  while  a  portion  of  the  distal  contributes  to  the 
formation  of  the  distal  fossa.  The  inner  boundary  of  the  cusp  is  formed 
by  the  buccal  groove,  the  distal  groove,  and  by  a  portion  of  the  disto- 
lingual groove. 

The  Mesiolingual  Cusp  (Fig.  89). — As  above  stated,  this  cusp  is 
frequently  the  largest  in  extent  of  surface,  and  is  somewhat  rounded, 
with  its  summit  poorly  defined.  The  ridges  which  descend  from  it 
correspond  in  name  and  number  to  those  of  the  buccal  cusps,  the  linguo- 


142  ANATOMY 

marginal  ridge  making  a  double  descent,  the  mesiolingual  ridge  descend- 
ing to  the  lingual  surface,  while  the  central  incline  is  marked  by  the 
mesiolingual  triangular  ridge,  which  ends  in  the  central  fossa.  Toward 
the  mesial  portion  of  the  cusp  one  or  more  small  ridges  are  frequently 
present,  extending  from  the  marginal  ridge  to  the  mesial  groove.  The 
distal  descent  of  the  marginal  ridge  is  bifurcated,  one  portion  making 
a  sweeping  curve  and  joining  the  transverse  ridge  from  the  buccal  cusp, 
forming  the  oblique  ridge  previously  referred  to,  the  other  portion  pass- 
ing in  a  distal  direction  and  ending  at  the  distolingual  groove.  The 
central  incline  of  this  cusp  forms  the  lingual  side  of  the  central  fossa, 
and  its  boundaries  are  outlined  by  the  mesial,  distal,  and  distolingual 
grooves. 

The  Distolingual  Cusp  (Fig.  89). — This  cusp  is  usually  the  smallest 
of  the  four;  it  is  triangular  in  outline,  with  the  summit  nearest  the  mesio- 
lingual portion.  The  ridges  which  descend  from  this  cusp  are  only  two 
in  number,  one  passing  in  a  mesial  direction  and  forming  a  portion  of  the 
linguomarginal  ridge,  the  other  passing  to  the  distal,  with  a  gradual 
buccal  curve,  to  join  the  distomarginal  ridge.  Of  the  two  remaining 
inclines,  one  looks  in  a  distolingual  direction,  presenting  a  surface  which 
is  smooth  and  rounded;  the  other,  sloping  by  a  broad  expanse  in  a  mesio- 
buccal  direction,  ending  in  the  distolingual  groove,  and  also  assisting  to 
form  the  distal  fossa.  This  latter  incline  is  often  crossed  by  small  sup- 
plemental grooves,  which  take  a  winding  course  from  the  base  to  the 
summit  of  the  incline.  The  inner  margin  or  outline  of  the  cusp  is  formed 
by  the  distolingual  groove. 

The  Fifth  Cusp  (Fig.  90). — Although  usually  referred  to  as  possessing 
but  four  cusps,  this  tooth  is  frequently  developed  with  five,  the  additional 
lobe  being  situated  on  the  lingual  side  of  the  mesiolingual  cusp,  about 
midway  between  its  summit  and  the  neck  of  the  tooth.  When  present, 
it  is  distinctly  separate  from  the  main  cusp  by  a  well-developed  groove — 
the  mesiolingual  groove.  Both  the  cusp  and  the  groove  may  be  more  or 
less  developed,  the  former  in  some  instances  assuming  dimensions  cor- 
responding to  that  of  the  distolingual  cusp,  and  the  latter  sometimes 
being  as  well  marked  as  the  distolingual  groove.  When  thus  pronounced, 
the  groove  begins  near  the  center  of  the  mesial  surface,  and  passes  ob- 
liquely toward  the  summit  of  the  mesiolingual  cusp,  before  reaching  which 
it  makes  an  abrupt  turn  rootward,  and  joins  the  lingual  terminal  of  the 
distolingual  groove,  this  union  frequently  resulting  in  a  well-defined  pit — 
the  lingual  pit.  This  cusp,  as  usually  found,  is  small  and  apparently 
without  function.     When  occurring  on  the  tooth  of  one  side,  it  is  usually 


UPPER   FIRST   MOLAR 


143 


present  on  the  corresponding  tooth  of  the  opposite  side.     It  is  seldom 
present  on  any  but  the  upper  first  molar. 

The  Fossae  and  Grooves  of  the  Occlusal  Surface  (Figs.  89  and 
go). — The  fossae  are  two  in  number — central  and  distal.  The  central 
fossa  occupies  a  position  near  the  center  of  the  surface,  and  is  formed  by 
the  central  incline  of  the  mesiobuccal,  distobuccal,  and  mesiolingual 
cusp,  which  usually  give  it  a  three-sided  form.  Connecting  the  three 
sides  of  the  fossa,  and  in  a  measure  assisting  in  its  construction,  is  the 


Buccal  Groove 


Distal  Groove 


Mesiobuccal 

Cusp 

Central  Fossa 

Mesiomar- 

ginal  Ridge 

The  Fifth 
Cusp 


Mesiolingual    Oblique 
Cusp  Ridge 

Fig.  90. — Upper  First  Molar,  Occlusal  Surface,  Strongly  Developed,  showing 
Presence  of  Fifth  Cusp. 


mesiomarginal  ridge  and  the  oblique  ridge.  The  depth  of  this  fossa, 
as  well  as  that  of  the  distal,  is  of  course  regulated  by  the  length  of  the 
cusps,  which  in  turn  is  much  influenced  by  the  temperamental  type  of  the 
tooth.  The  floor  of  the  fossa  is  deeply  marked  by  two  of  the  grooves 
of  development — the  mesial  groove  and  the  buccal  groove.  The  former 
begins  on  the  mesial  surface,  passes  over  the  mesiomarginal  ridge,  and 
continues  in  an  irregular  line  to  the  floor  of  the  fossa;  the  latter,  begin- 
ning near  the  center  of  the  buccal  surface,  enters  the  fossa  by  crossing 
the  buccomarginal  ridge  near  the  center  of  its  length,  and  also  ends  in 
the  central  pit  of  the  central  fossa.  As  previously  referred  to,  the  union 
of  these  two  grooves  forms  the  mesiobuccal  triangular  groove.  From 
the  central  pit  of  this  fossa  another  groove  is  given  off — the  distal  groove. 
It  is  usually  well  defined  at  its  beginning,  but  as  it  passes  over  the  oblique 
ridge  it  is  generally  partly  obliterated,  although  occasionally  being  so 
marked  as  to  divide  this  ridge.  The  distal  fossa  is  much  smaller  than 
the  central,  and  is  of  an  entirely  different  form.  Its  walls  are  principally 
formed  by  the  distolingual  incline  of  the  oblique  ridge,  and  the  mesio- 
buccal incline  of  the  distolingual  cusp;  a  portion  of  the  distomarginal 


144 


ANATOMY 


Mesiobuccal 
Root 


ridge  and  the  distal  incline  of  the  distobuccal  cusp  also  assist  in  its  forma- 
tion. Like  the  central  fossa,  its  sides  are  more  or  less  irregular,  from 
the  presence  of  various  grooves  and  ridges  in  its  vicinity.  The  greatest 
length  of  the  fossa  is  in  a  distolingual  direction,  and  it  is  traversed  by 
a  deep  developmental  groove — the  distolingual  groove.  When  the  distal 
groove  crosses  the  oblique  ridge,  it  usually  extends  to  the  floor  of  this 
fossa. 

The  Buccal  Surface  of  the  Crown  (Fig.  91). — This  surface,  which 
is  the  result  of  a  union  between  the  mesial  and  distal  developmental 

lobes,  may  be  divided  into  a 
mesial  and  a  distal  half.  These 
two  portions  are  quite  similar 
in  outline,  and  are  separated 
Lingual  Root  from  each  other  by  the  buccal 
groove,  which  usually  ends  near 
|the  center  or  about  half-way  to 
the  cervical  line  in  a  decided 
pit — the  buccal  pit.  In  some 
instances  this  groove  is  con- 
tinued to  the  cervical  line,  or 
even  beyond  this  to  the  bifur- 
cation of  the  roots.  Both  the 
mesial  and  distal  half  are  pro- 
vided with  a  longitudinal  ridge 
(the  buccal  ridges) — one  the 
mesiobuccal  ridge  and  the  other 
the  distobuccal  ridge.  These 
are  similarly  formed  and  descend  from  the  summits  of  the  respective  cusps, 
at  which  point  they  are  usually  well  defined,  but  gradually  disappear  as 
they  pass  toward  the  cervical  line.  The  location  of  the  buccal  groove  being 
a  little  to  the  distal  of  the  center  of  the  surface,  gives  to. the  mesial  portion 
a  somewhat  greater  extent  than  the  distal.  The  margins  of  the  surface, 
which  form  an  irregular  quadrilateral,  are  the  mesial,  distal,  occlusal,  and 
cervical.  The  mesial  and  distal  margins  are  rounded,  and  gradually  con- 
verge as  they  pass  rootward,  making  the  average  diameter  of  the  surface 
about  one-fourth  less  at  the  cervical  line  than  at  the  base  of  the  cusps. 
In  some  instances  these  margins  are  slightly  concave  over  their  cervical 
portion,  and  convex  on  approaching  the  occlusal  margins;  or  the  mesial 
may  be  concave  and  the  distal  convex  throughout  their  entire  length; 
in  some  types  they  appear  as  straight  lines  and  are  parallel  with  each 


Distobucca 
Cusp 


Mesiobuccal 
Cusp 


Fig.  91.- 


Buccal  Groove 

-Upper  First  Molar,  Buccal 
Surface. 


UPPER    FIRST    MOLAR 


145 


other.  The  occlusal  margin  is  formed  by  the  marginal  ridges  as  they 
pass  over  the  two  buccal  cusps,  being  in  the  form  of  the  letter  W.  The 
cervical  margin  is  usually  a  direct  line  drawn  around  the  circumference 
of  the  tooth,  but  in  some  instances  deviating  slightly  from  this.  Immedi- 
ately below  the  cervical  line,  and  conforming  to  its  general  direction,  is 
a  rounded  fold  of  enamel — the  cervical  ridge. 

Lingual  Surface  of  the  Crown*  (Fig.  92).— Like  the  buccal  sur- 
face, this  surface  is  developed  from  two  lobes — the  mesio-  and  disto- 
lingual  lobes — the  line  of  union  between  the  two  being  recorded  by  a  well- 
defined  groove — the  lingual 
groove.  This  groove,  which 
is  a  continuation  of  the  dis- 
tolingual  groove  of  the  occlusal 
surface,  usually  ends  near  the 
center  of  the  lingual  surface  in 
a  well-defined  pit — the  lingual 
pit — or  it  may  continue  root- 
ward  and  gradually  disappear. 
It  is  located  a  little  to  the 
distal  of  the  center  of  the  sur- 
face, thus  making  the  mesial 
a  trifle  larger  than  the  distal 
portion.  The  mesial  half  of 
the  surface  is  smooth  and 
convex;  the  lingual  incline  of 

the  mesiolingual  cusp  is  seldom  provided  with  a  well-defined  ridge, 
although  usually  referred  to  as  the  mesiolingual  ridge.  The  distal  half 
of  the  surface  is  also  smooth  and  rounded,  with  the  mesiodistal  convexity 
much  more  marked  than  that  of  the  mesial  lobe.  The  cervical  ridge  is 
seldom  so  pronouced  as  that  of  the  buccal  surface,  but  the  enamel 
frequently  makes  a  sudden  dip  at  this  point  to  meet  the  cementum  of 
the  root.  That  portion  of  the  surface  immediately  below  the  cervical 
ridge  is  smooth  and  unbroken,  slightly  convex  in  the  direction  of  the 
long  axis  of  the  tooth,  and  flattened  or  slightly  convex  from  mesial  to 
distal.  The  margins  cf  the  surface  are  the  mesial,  distal,  occlusal,  and 
cervical.  The  surface  passes  so  gradually  into  the  mesial  and  distal 
that  it  is  somewhat  difficult  to  define  these  margins.  In  general,  the 
margins  converge  slightly  in  the  direction  of  the  root.     Both  the  occlusal 


Distolingual 
Angle 


Fig.  92.- 


Cervical 
Line 


Mesial 
Groove 


Lingual  Ridge 
Mesiolingual  Cusp 

-Upper  First  Molar,  Lingual 
Surface. 


*  When  the  fifth  cusp  is  present,  the  anatomy  of  the  mesial  half  of  this  surface  is  some- 
what more  complex. 


146  ANATOMY 

and  cervical  margins  are  similar  to  the  corresponding  margins  of  the 
buccal  surface. 

The  Mesial  Surface  of  the  Crown  (Fig.  93). — This  surface  is 
almost  an  unbroken  plane,  being  smooth  and  flat.  In  some  instances 
it  is  crossed  near  the  center  of  its  occlusal  margin  by  a  continuation  of 
the  mesial  groove,  but  this  is  seldom  so  pronounced  as  to  divide  the 
surface.  The  occlusal  third  of  the  surface  is  inclined  to  a  slight  general 
convexity,  providing  a  point  of  contact  for  the  distal  surface  of  the  second 

bicuspid,    but    between    this 

and  the  cervical  line  there  is 

often  a  slight  concavity.     The 

,    Mesiobuc-  margins  of  the  surface  are  the 

Lingual     H  «■        caljRoot 

Root  occlusal,  buccal,  lingual,  and 

cervical.     The  first  named  is 

formed  by  the  mesiomarginal 

cervical    I  B  ridge  of  the  occlusal  surface, 

and  is  concave  in  the  direc- 
tion of  the  root.     The  buccal 

Mesiolingual  <■      I     Mesiobuc-  j        v  1 

Angle  iL-^^^^^M     I    caf Angle    and     lingual      margins     are 

rounded,  and,  unlike  the  lat- 

Fig.  93.-Upper  First  Molar,  Mesial  eraJ     margins     0f     the     buccal 

•  Surface.  ° 

and  lingual  surfaces,  diverge 
in  the  direction  of  the  roots.  The  cervical  margin  is  slightly  concave  in 
the  direction  of  the  occlusal  surface,  and  its  length  is  much  greater  than 
that  of  any  other  margin  of  the  crown.  This  surface  is  more  extensive 
than  either  the  buccal,  lingual,  or  distal,  and  is  about  equal  to  that  of 
the  occlusal.  When  the  fifth  cusp  is  present  it  alters  the  form  of  the 
lingual  margin  of  this  surface  by  crossing  it  near  the  center,  and  ex- 
tending for  some  little  distance  on  the  face  of  the  surface. 

The  Distal  Surface  of  the  Crown  (Fig.  94). — Taken  in  its  entirety, 
this  surface  usually  presents  a  general  convexity.  The  lingual  half  of 
the  surface  is  usually  somewhat  more  prominent  than  the  buccal,  the 
latter  being  flattened  and  frequently  slightly  concave,  particularly  near 
the  cervical  portion.  In  some  instances  the  surface  is  traversed  by  a 
continuation  of  the  distolingual  groove,  which,  after  passing  over  the 
distomarginal  ridge,  is  continued  in  a  longitudinal  direction,  dividing 
the  surface  into  two  equal  parts.  Not  infrequently  this  groove,  instead 
of  existing  as  such,  is  represented  as  a  shallow  depression,  often  extend- 
ing to  the  bifurcation  of  the  roots.     The  margins  of  the  surface  are  four 


UPPER    FIRST    MOLAR 


J47 


Lingual 
Root 


Mesiobuc- 
cal  Root 

Distobuc- 
cal  Root 


in  number:  the  occlusal,  which  closely  resembles  the  corresponding 
margin  of  the  mesial  surface;  the  buccal,  which  is  not  well  defined;  the 
lingual,  somewhat  angular;  and  the  cervical,  formed  by  the  cervical  line. 

The  Neck  of  the  Tooth. — When  looking  upon  the  buccal  surface, 
the  constricted  portion  forming  the  neck  is  greatest  at  a  point  immediately 
above  the  cervical  line.  Viewed  in  this  direction  the  crown  is  usually 
bell-shaped,  and  both  the  crown  and  the  base  of  the  roots  assist  in  pro- 
ducing the  neck.  Viewed  from  a  lingual  direction,  the  neck  is  a  dis- 
tinctive feature,  but  is  seldom  so  marked  as  when  examined  from 
the  opposite  side.  When  studied 
from  either  a  mesial  or  a  distal 
aspect,  the  neck  appears  above 
the  cervical  line,  the  prominent 
fold  of  enamel  immediately  adja- 
cent to  this  line  forcing  the  neck 
rootward. 

The  Roots  of  the  Upper  First 
Molar. — The  roots  are  three  in 
number,  two  of  which  are  on 
the  buccal  side,  and  are,  there- 
fore, called  mesiobuccal  and  disto- 
buccal  roots,  and  one  on  the  lingual 
side,  known  as  the  lingual  root. 
These  three  roots  are  given  off 
from    a   common   base,    which   is 

sometimes  referred  to  as  the  root,  while  those  parts  above  the  point  of 
trifurcation  are  considered  as  root-branches.  The  number,  location, 
and  form  of  the  roots  of  this  tooth  are,  perhaps,  more  constant  than  those 
found  in  connection  with  any  other  cuspidate  tooth.  The  common  base 
from  which  the  roots  are  given  off  is  similar  in  contour  to  the  crown  of 
the  tooth,  excepting  in  those  cases  in  which  the  form  of  the  root  is  carried 
over  this  base  to  meet  the  neck  of  the  tooth. 

The  mesiobuccal  root  (Fig.  93)  is  flattened  from  mesial  to  distal,  broad 
at  its  base  from  buccal  to  lingual,  from  which  point  it  gradually  tapers 
to  the  apex.  At  the  base  the  mesiodistal  measurement  is  less  than  one- 
third  that  of  the  buccolingual.  In  its  course  it  is  first  inclined  to  the 
mesial,  but  after  reaching  the  center  of  its  length  it  makes  a  decided 
distal  curve,  which  looks  almost  directly  to  the  distal.  The  mesial  side 
of  this  root  is  decidedly  flattened  at  its  base,  but  as  the  center  of  the 
surface  is  reached  a  shallow  longitudinal  groove  is  present,  which  is 


Disto- 
lingual 
Cusp 


Fig. 


s 


Distobuc- 
cal  Cusp 


91. 


Distolingual  Groove 

-Upper  First  Molar,  Distal 
Surface. 


I48  ANATOMY 

continued  to  the  region  of  the  apex.  The  distal  side  is  also  possessed 
of  a  similar  groove,  which  extends  throughout  its  entire  length.  Both 
the  buccal  and  lingual  sides  of  the  root  are  smoothly  convex,  the  latter 
being  only  about  half  the  width  of  the  former. 

The  distobuccal  root  (Fig.  94)  is  much  the  smallest  of  the  three,  and, 
while  inclined  to  flatness  on  its  mesial  and  distal  sides,  it  is  much  more 
rounded  than  the  mesial  root.  The  mesial  side  is  provided  with  a  slight 
longitudinal  groove,  and  in  rare  instances  a  similar  groove  exists  on  the 
distal  side.  The  buccal  and  lingual  sides  are  similar  to  those  of  the 
mesial  root.  The  root  is  generally  straight,  and  tapers  gradually  from 
base  to  apex,  ending  in  a  rounded  point. 

The  lingual  root  (Fig.  94)  is  usually  the  largest  and  longest  of  the 
three,  and  is  more  rounded  in  form  than  either  of  the  buccal  roots.  The 
lingual  surface  is  inclined  to  flatness  near  its  base,  and  is  provided  with 
a  well-defined  longitudinal  groove,  which  is  sometimes  independently 
formed,  while  at  others  it  is  present  as  a  continuation  of  the  lingual 
groove.  This  root  being  the  only  one  given  off  from  the  lingual  side  of 
the  tooth,  is  constructed  with  a  mesiodistal  measurement  about  equal 
to  that  of  the  base  of  the  crown  at  this  point.  From  its  place  of  beginning 
it  passes  first  in  a  lingual  and  then  in  a  buccal  direction,  forming  a  long 
curve  and  ending  in  a  sharp-pointed  apex. 

Bilious  Type. — The  upper  first  molar  of  this  temperament  is  mani- 
fest by  a  crown  with  angles  well  produced,  the  marginal  ridges  and 
cutting-edges  of  the  cusps  bold  and  well  marked.  The  cusps  are  of 
medium  length,  with  summits  angular  and  pointed.  The  developmental 
grooves  are  deep  and  often  sulcate,  and  numerous  supplemental  grooves 
are  found  upon  the  occlusal  surface.  The  longitudinal  and  transverse 
measurements  of  the  crown  are  about  equal,  and  when  viewed  upon  the 
occlusal  surface,  the  angular  nature  of  its  anatomy  is  noted  as  a  distinc- 
tive feature.  The  neck  is  fairly  well  developed,  giving  a  slight  bell- 
shape  to  the  crown.  The  cervical  line  is  made  up  of  angles  rather  than 
curves,  and  the  roots  are  long  and  straight. 

Nervous  Type. — Like  the  teeth  previously  described  under  this 
class,  the  crown  of  this  tooth  is  of  greater  longitudinal  than  transverse 
extent;  the  neck  is  especially  well  formed,  producing  a  decided  bell- 
shape  to  the  crown.  The  cusps  are  long  and  penetrating,  the  marginal 
ridges  sharply  defined,  as  are  also  those  ridges  upon  the  central  incline 
of  the  cusps.  The  grooves  of  development  are  decided  and  frequently 
sulcate.  The  buccal  surface  is  rounded  and  smooth,  with  the  buccal 
groove  extending  well  toward  the  cervical   line.     The  lingual  surface 


UPPER    FIRST    MOLAR  149 

also  presents  a  general  convexity,  and  is  usually  divided  by  the  lingual 
groove.  The  mesial  surface  is  convex  over  its  cervical  third,  and  the 
occlusal  margin  is  a  decided  convex  ridge,  serving  as  a  point  of  contact 
for  the  adjoining  tooth,  and  thus  forming  the  characteristic  V-shape 
common  to  this  temperament.  It  is  in  this  type  that  the  fifth  cusp  is 
most  frequently  present.  The  cervical  line  is  much  curved,  and  the 
roots  are  slim  and  frail. 

Sanguineous  Type. — The  crown  of  the  upper  first  molar  of  this 
type  usually  presents  a  slightly  greater  longitudinal  than  transverse 
extent.  The  angles  of  the  crown  are  poorly  formed,  being  rounded  and 
smooth.  The  cusps  are  of  moderate  length,  and  are  rounded  in  their 
nature;  the  marginal  ridges,  as  well  as  those  ridges  of  the  central  incline 
of  the  cusps,  are  less  distinct  than  either  of  the  forms  previously  described. 
The  buccal  and  lingual  surfaces  are  convex  and  seldom  broken  by  grooves; 
the  mesial  and  distal  surfaces  are  convex  in  every  direction,  throwing 
the  point  of  contact  with  adjoining  teeth  near  the  center  of  the  surface. 
The  roots  are  inclined  to  be  large  and  oval  in  form,  while  the  cervical 
line  is  a  series  of  long  curves. 

Lymphatic  Type. — In  this  temperament  the  crown  is  much  less 
in  its  longitudinal  than  transverse  measurement.  The  neck  of  the  tooth 
is  poorly  defined,  the  crown  passing  into  the  root-base  without  a  marked 
constriction.  The  mesial  and  distal  surfaces  are  flattened  and  nearly 
parallel  with  each  other,  providing  a  broad  contact  surface.  The  buccal 
and  lingual  surfaces  each  present  a  marked  general  convexity,  the  latter 
being  frequently  broken  by  the  distolingual  groove.  The  tooth  is  pro- 
vided with  cusps  which  are  short  and  heavy-set;  the  marginal  ridges, 
as  well  as  all  the  ridges  common  to  the  occlusal  surfaces,  are  poorly  defined. 
The  developmental  grooves  are  shallow  and  terminate  abruptly.  There 
is  but  little  curvature  to  the  cervical  line,  and  the  roots  are  short,  heavy- 
set,  and  inclined  to  cluster  together. 


IS© 


ANATOMY 


UPPER  SECOND  MOLAR. 


e*~  t&  ft  F  f 


7th  year 


8th  year         9th  year 


10th  year 

Fig.  95. 


nth  year        12th  year        16th  year 


Calcification  Begins,  from  Four  Centers,  about  the  Fifth  Year. 
Calcification  Completed,  Sixteenth  to  Eighteenth  Year. 
Erupted,  Twelfth  to  Fourteenth  Year. 
Average  Length  of  Crown,  .28. 

Average  Length  of  Root,  .51. 

Average  Length  Over  All,  .79. 

Calcification  in  this  tooth  takes  place  in  precisely  the  same  manner 
as  that  of  the  first  molar,  but  the  formative  process  is  much  later  in  begin- 
ning, the  lime-salts  commencing  to  accumulate  in  the  four  separate  lobes 
about  the  fifth  year.  At  the  beginning  of  the  sixth  year  the  formation 
of  the  cusps  is  completed,  soon  after  which  they  coalesce  and  the  occlusal 
surface  of  the  crown  is  established.  At  the  beginning  of  the  eighth 
year  fully  two-thirds  of  the  crown  is  calcified,  and  the  following  year 
the  crown  and  neck  are  completed  and  the  root-base  outlined.  By  the 
tenth  year  the  beginning  of  separate  root-development  is  observed;  at 
the  twelfth  year,  or  at  the  time  of  eruption,  the  roots  are  formed  to  about 
one-half  of  their  completed  length,  the  process  continuing  until  the  six- 
teenth or  seventeenth  year,  when  calcification  is  completed  and  the  root 
apices  formed  (Fig.  95).  In  many  respects  this  tooth  closely  resembles 
the  first  molar  previously  described,  the  crown  presenting  the  same 
number  of  surfaces  similarly  named,  and  also  being  provided  with 
the  same  number  of  roots.  Notwithstanding  this  fact,  there  are  a 
number  of  ways  in  which  they  are  at  variance.  The  crown  of  the  sec- 
ond molar  is  smaller  than  that  of  the  first,  and  the  quadrilateral  out- 
line common  to  the  first  molar  is  much  compressed  and  broken  in  the 
second.  The  distal  cusps  are  much  smaller  proportionately  than  the 
mesial  cusps,  this  being  particularly  true  of  the  distolingual  cusp.    This 


UPPER    SECOND    MOLAR 


151 


reduction  in  size  of  the  distal  cusps  gives  to  that  portion  of  the  occlusal 
surface  a  slight  distal  incline. 

Occlusal  Surface  of  the  Crown  (Fig.  96). — The  general  contour 
of  the  crown  is  best  studied  by  viewing  it  directly  upon  the  occlusal 
surface;  this  aspect  also  shows  to  best  advantage  the  difference  in  form 
between  this  and  the  first  molar,  as  shown  in  figure  90.  The  mesial 
and  lingual  outlines  closely  resemble  the  corresponding  outlines  on 
the  first  molar,  but  the  buccal  and  distal  are  much  at  variance,  the 
former  passing  into  the  latter  without  a  distinct  line  of  demarcation 
existing  between  the  two,  this  gradual  blending  of  one  into  the  other 
being  at  the  expense  of  the  distobuccal  angle  of  the  crown,  which  is  poorly 

Buccal  Groove 


Distobuccal  Cusp 

Distal  Fossa 

Distolingual  Groove 


Mesiobuccal 
Triangular  Ridge 

Mesial  Groove 
Mesiomarginal  Ridge 

Mesiolingual  Cusp 


Distal  Incline  of 
Mesiolingual  Cusp 


Fig.  96. — Upper  Second  Molar,  Occlusal  Surface. 


developed.  The  crown  is  much  compressed  in  a  distobuccal-mesio- 
lingual  direction,  making  this  measurement  of  the  occlusal  surface  about 
one-third  less  than  the  mesiobuccal-distolingual  measurement.  The 
cusps  are  much  inclined  to  cluster  toward  the  center  of  the  surface,  this 
being  especially  true  of  those  on  the  lingual  half. 

Marginal  Ridges  of  the  Occlusal  Surface  (Fig.  96). — Like  the  occlusal 
surface  of  the  upper  first  molar,  this  surface  of  the  second  molar  is 
bounded  by  four  marginal  ridges — the  mesial,  distal,  buccal,  and  lingual. 
They  are  usually  less  marked  than  those  found  on  the  first  molar,  and 
are  much  more  variable  in  their  individual  anatomy.  The  mesiomarginal 
ridge  extends  from  the  summit  of  the  mesiobuccal  cusp  to  the  summit 
of  the  mesiolingual  cusp.  It  is  concave  in  the  direction  of  the  body  of 
the  crown,  and  is  broken  near  its  central  portion  by  the  mesial  groove. 
In  some  instances  one  or  more  small  supplemental  grooves  are  found 


152  ANATOMY 

to  cross  it.  Compared  with  the  mesiomarginal  ridge  of  the  first  molar, 
its  length  is  much  less  and  the  convexity  not  so  pronounced.  The 
distomarginal  ridge,  owing  to  the  variation  in  form  and  size  of  the  distal 
cusps,  is  difficult  to  describe  definitely;  suffice  it  to  say  that  it  extends 
from  the  summit  of  the  distobuccal  to  the  summit  of  the  distolingual 
cusp.  The  concavity  is  V-shaped,  and  is  usually  crossed  near  the  center 
by  the  distolingual  groove.  In  some  instances  the  distolingual  cusp 
is  almost  wanting,  in  others  the  distobuccal  is  but  little  developed;  when 
either  of  these  conditions  is  present,  the  marginal  ridge  is  extended 
either  to  the  buccal  or  to  the  lingual,  in  a  measure  taking  the  place  of 
the  missing  cusp.  The  mesial  half  of  the  buccomar ginql  ridge  closely 
resembles  the  corresponding  margin  of  the  first  molar;  beginning  at 
the  mesiobuccal  angle  it  ascends  to  the  summit  of  the  mesiobuccal 
cusp,  after  which  it  descends  by  a  longer  incline  to  the  buccal  groove. 
The  distal  half  of  the  ridge,  unlike  that  of  the  first  molar,  presents  much 
variety,  its  form  being  controlled  by  the  character  and  position  of  the 
distobuccal  cusp,  usually  small.  As  most  frequently  observed,  it  ascends 
to  the  summit  of  the  cusp,  and  in  so  doing  it  presents  a  decided  lingual 
inclination.  In  passing  down  the  distal  incline  the  lingual  inclination 
is  increased  and  gradually  passes  into  the  distomarginal  ridge.  Branch- 
ing off  from  the  mesiomarginal  ridge,  the  linguomar ginal  ridge  ascends 
to  the  summit  of  the  mesiolingual  cusp  and  descends  by  a  much  shorter 
incline  to  the  distolingual  groove.  This  portion  of  the  margin  is  thrown 
well  toward  the  center  of  the  surface,  the  location  of  the  cusp  carrying 
it  to  that  point.  Like  the  distal  half  of  the  buccal  margin,  the  outline 
of  the  distal  half  of  this  margin  is  controlled  by  the  position  and  form  of 
the  distolingual  cusp.  In  the  majority  of  cases,  when  the  cusp  is  moder- 
ately strong,  the  ascent  from  the  distobuccal  groove  to  the  summit  of  the 
cusp  is  short  and  abrupt,  the  descent  being  somewhat  more  gradual, 
and  with  a  decided  buccal  inclination  it  passes  into  the  distomarginal 
ridge,  or  ends  abruptly  at  the  distal  end  of  the  distolingual  groove. 

The  Cusps  and  Ridges  (Fig.  96). — This  tooth  is  provided  with 
four  lobes  or  cusps,  two  of  which  are  located  on  the  buccal,  and  two  on 
the  lingual  side.  They  are  usually  smaller  and  less  angular  than  the 
cusps  of  the  first  molar.  This  is  particularly  true  of  both  the  distal 
cusps,  and  especially  of  the  distolingual  cusp,  which  is  often  quite  diminu- 
tive and  occasionally  entirely  wanting.  When  this  latter  condition 
exists,  the  lingual  half  of  the  surface  is  for  the  most  part  occupied  by 
what  would  otherwise  be  the  mesiolingual  cusp,  the  absence  of  the 
distal  cusp  permitting  the  distolingual  groove  to  occupy  a  position  near 


UPPER    SECOND    MOLAR  1 53 

the  extreme  distolingual  angle,  that  portion  of  the  surface  which  is 
distal  to  the  groove  being  a  portion  of  the  distomarginal  ridge. 

The  Mesiobuccal  Cusp  (Fig.  96). — Like  the  corresponding  cusp  of 
the  first  molar,  this  cusp  is  usually  the  longest  of  the  four,  and  in  many 
instances  covers  a  greater  extent  of  surface  than  any  of  the  others.  Its 
base  is  outlined  by  the  buccal  and  mesial  grooves,  the  two  together  form- 
ing the  mesiobuccal  triangular  groove.  Descending  from  its  summit 
to  the  buccal  surface  is  the  mesiobuccal  ridge;  the  marginal  ridge  makes 
a  double  descent,  one  in  a  mesial  and  one  in  a  distal  direction,  while 
sloping  toward  the  central  fossa  is  the  mesiobuccal  triangular  ridge. 
The  cusp  is  seldom  traversed  by  supplemental  grooves  such  as  are  found 
on  the  corresponding  cusp  of  the  first  molar. 

The  Distobuccal  Cusp  (Fig.  96). — As  previously  stated,  this  cusp  is 
not  constant  in  its  form;  in  some  instances  it  is  bold  and  well  produced, 
corresponding  closely  to  the  mesiobuccal  cusp  just  described.  When 
thus  pronounced  it  is  possessed  of  ridges,  and  bounded  by  grooves  which 
are  similar  to  those  described  in  connection  with  the  first  molar.  More 
frequently  the  cusp  is  much  rounded,  its  summit  being  carried  well 
toward  the  center  of  the  surface.  When  this  formation  exists,  the  buccal 
ridge  is  absent,  the  marginal  ridges  short  and  rounded;  the  distobuccal 
triangular  ridge  which  descends  from  it  toward  the  center  of  the  crown 
is  short  and  heavy  set. 

The  Mesiolingual  Cusp  (Fig.  96). — In  the  majority  of  instances  this  is 
the  largest  cusp,  particularly  when  there  is  a  degenerate  tendency  in  the 
distolingual  cusp.  Descending  from  its  summit  are  a  number  of  ridges, 
the  marginal  ridges  being  given  off  as  already  described,  the  mesiolingual 
triangular  ridge  descending  the  central  incline  to  the  central  fossa,  and 
when  the  cusp  has  an  additional  mesiodistal  extent  by  the  presence  of  a 
diminutive  distal  cusp,  other  ridges  descend  in  the  same  direction.  The 
lingual  descent  of  the  cusp  is  smooth  and  more  rounded  than  the  corre- 
sponding surface  of  the  first  molar,  and  is  seldom  elevated  in  the  form  of  a 
definite  ridge.  The  central  outline  of  this  cusp  is  marked  by  the  mesial, 
the  distal,  and  the  distolingual  grooves. 

The  Distolingual  Cusp  (Fig.  96). — In  no  other  cusp  do  we  find  such 
a  diversity  of  form  as  in  the  distolingual  cusp  of  the  upper  second  molar. 
In  some  instances  it  is  fully  as  prominent  as  its  neighbor  just  described, 
in  others  appearing  as  a  mere  fold  of  enamel,  and  it  is  not  uncommon 
to  find  it  entirely  wanting,  the  distomarginal  ridge  extending  to  occupy 
a  portion  of  the  space  which  it  should  claim.  Deductions  might  be 
drawn  from  an  average  between  these  two  extremes,  wherein  the  existing 


x54 


ANATOMY 


cusp  would  be  much  smaller  than  any  of  the  others,  the  summit  rounded 
rather  than  sharp,  but  with  a  decided  inclination  to  occupy  the  extreme 
distolingual  angle  of  the  surface,  in  this  latter  respect  differing  from  the 
distobuccal  cusp.  The  mesial  and  buccal  outlines  of  the  cusp  are  formed 
by  the  distolingual  groove,  and  its  mesiolingual  incline  contributes  to 
the  formation  of  the  distal  fossa. 

The  Fossae  and  Grooves  of  the  Occlusal  Surface  (Fig.  96). — 
These  in  name,  number,  and  general  form  are  similar  to  those  of  the 
first  molar.  The  central  fossa  is  never,  strictly  speaking,  in  the  center 
of  the  surface,  and  is  formed  by  the  central  incline  of  the  mesiobuccal, 
distobuccal,  and  mesiolingual  cusps.  It  is  seldom  so  deep  as  the  central 
fossa  of  the  first  molar.  The  distal  fossa  is  more  or  less  pronounced, 
its  size  and  position  being  controlled  by  the  extent  of  development  in 
the  distolingual  cusp.  The  distolingual  groove,  which  usually  crosses 
the  lingual  surface  of  the  first  molar  near  its  center,  is  not  constant  in 
its  location  on  this  tooth,  in  some  cases  being  near  the  center,  in  others 
near  the  distolingual  angle  of  the  crown.  The 
buccal  groove  is  never  constant  in  its  location, 
usually  crossing  the  buccomarginal  ridge  and  pass- 
ing over  the  buccal  surface  near  its  mesiodistal 
center,  but  it  is  not  uncommon  to  find  it  forced  to 
the  distal  by  a  diminution  in  the  size  of  the  distal 
cusp. 

Buccal  Surface  of  the  Crown  (Fig.  97). — 
The  most  constant  difference  between  this  and  the 
corresponding  surface  of  the  upper  first  molar  is 
the  wandering  location  of  the  buccal  groove. 
While  in  the  majority  of  instances  it  may  be  found 
near  the  mesiodistal  center  of  the  surface,  it  is  not 
uncommon  to  find  it  passing  over  the  distal  third, 
or  even  as  far  posterior  as  the  distobuccal  angle. 
In  general,  the  surface  is  somewhat  more  convex  and  necessarily  less 
extensive  than  the  buccal  surface  of  the  first  molar.  The  buccal  ridges 
which  descend  from  the  summit  of  the  two  buccal  cusps  are  seldom  so 
marked  as  those  on  the  first  molar,  and  in  many  instances  the  distal 
ridge  is  wanting.  The  distal  half  of  the  surface  frequently  passes  into 
the  distal  surface,  by  a  long  gradual  sweep,  there  being  no  line  of  demar- 
cation between  the  two. 

The  Lingual  Surface  of  the  Crown   (Fig.  98). — In  keeping  with 
the  other  surfaces  just  described,  the  lingual  surface  differs  from  the 


Fig.  97.  —  Upper 
Second  Molar,  Buc- 
cal Surface. 


UPPER    SECOND    MOLAR 


J55 


Cervical 
Line 


Disiomar- 
ginal?Ridge 


Lingual  Groove,  ending 
in  Lingual  Pit 

Fig.  98. — Upper  Second  Molar,  Lingual 
Surface. 


corresponding  surface  of  the  first  molar  in  that  it  presents  a  greater 
general  convexity.  This  is  particularly  true  in  passing  from  the  cervical 
line  to  the  occlusal  surface.  The  lingual  groove  is  also  less  constant 
in  its  location.  In  most  in- 
stances it  is  to  be  found  a 
little  to  the  distal  of  the  cen- 
ter, in  others  being  as  far 
posterior  as  the  extreme  distal 
third  of  the  surface,  and  in 
rare  instances  it  is  entirely 
wanting.  The  general  char- 
acter of  this  surface,  which  is 
smooth  and  convex,  is  seldom 
broken  by  the  presence  of 
well-defined  ridges,  such  as 
are  usually  found  descending 
from  the  lingual  cusps  of  the 
first  molar.  The  mesial,  dis- 
tal, and  buccal  surfaces,  as  well  as  the  surfaces  under  consideration,  are 
proportionately  smaller  than  those  of  the  first  molar,  and,  while  this  refers 
to  both  the  transverse  and  longitudinal  measurements,  it  is  particularly 
applicable  to  the  latter. 

The  Mesial  Surface  of  the 
Crown  (Fig.  99). — Aside  from 
this  surface  being  of  less  extent 
Mesiobuc-  than  the  corresponding  surface  of 
the  first  molar,  there  are  no  other 
differences  of  importance.  In 
many  instances,  however,  there 
is  a  decided  tendency  for  the 
surface  to  be  concave  from 
buccal  to  lingual,  the  convex 
distal  surface  of  the  first  molar 
closely  fitting  into  this  concavity. 
Another  variation  which  is  fre- 
quently observed  is  that  of  the 
longer  and  more  gradual  sweep  which  it  takes  in  passing  into  the  lingual 
surface. 

The  Distal  Surface  of  the  Crown  (Fig.  100). — This  differs  from 
the  distal  surface  of  the  first  molar  principally  in  its  more  pronounced 


Mesiolingual 
Angle 


Mesiobuc- 
cal  Angle 


Fig.  99. — Upper  Second  Molar,  Mesial 
Surface. 


156 


AN  \  ni\i\ 


convexity.  Its  general  form  is  also  much  influenced  by  the  nature  of 
the  two  distal  cusps.  If  one  or  the  other  of  these  is  sparingly  developed, 
either  the  buccal  or  lingual  hall"  of  the  surface,  as-  the  case  may  be,  is 
quickly  rounded  off  to  pass  into  the  deficient  lobe. 

The  Angles  of  the  Crown. — The  increased  inclination  for  the 
crown  of  this  tooth  to  general  convexity  dispels,  in  a  measure,  the  presence 
of  angles,  as  such,  in  correspondence  to  the  four  corners  of  the  first 
molar.  In  some  instances  the  crown  is  represented  as  a  fairly  well- 
formed  quadrilateral,  in  which  case  the  angles  are  well  defined,  but 

usually  this  outline  is  so  much 
broken  by  a  mesiodistal  com- 
pression that  the  angular  form 
of  the  crown  is  entirely  abolished. 
But,  whatever  the  form  of  the 
crown  may  be,  it  is  well  to  ad- 
here to  the  commonly  accepted 
term,  and  speak  of  that  point  at 
which  the  sides  of  the  crown 
unite  as  the  angles,  each  being 
named  in  accordance  with  its 
location. 

The  Neck  of  the  Tooth.— 
The  principal  variation  between 
the  neck  of  this  tooth  and  that 
of  the  first  molar,  is  that  produced  by  the  greater  general  convexity  of 
the  crown,  which  contributes  to  the  production  of  a  neck  much  more 
constricted.  There  is  also  a  greater  variety  in  the  contour  of  the  neck, 
incident  to  the  variation  in  the  general  outline  of  the  crown. 

The  Roots  of  the  Upper  Second  Molar. — These  are  the  same 
in  name  and  number  as  those  of  the  first  molar — two  buccal  and  one 
lingual.  In  many  respects  they  differ  from  the  roots  of  the  first  molar. 
They  are  much  smaller,  frequently  inclined  to  cluster  together,  and  are 
often  fused,  in  some  instances,  all  three  being  united,  in  others  the  union 
existing  between  but  two.  When  isolated,  each  root  usually  presents  a 
decided  distal  curve  near  its  apical  third.  When  the  crown  is  flattened 
from  mesial  to  distal,  as  before  described,  the  distobuccal  root  is  forced 
to  occupy  a  position  much  more  to  the  lingual  than  that  assumed  by 
the  mesiobuccal  root.  The  lingual  groove  seldom  passes  over  the  lingual 
root,  as  observed  on  the  first  molar. 


Fig.  ioo. 


-Upper  Second  Molar,  Distal 
Surface. 


UPPER    THIRD    MOLAR  1 57 


UPPER  THIRD  MOLAR. 


m        w* 


10th  year  nth  year  12th  year  14th  year  1 8th  year 

Fig.  ioi. 

Calcification  Begins,  Ninth  Year. 
Calcification  Completed,  Eighteenth  to  Twentieth  Year. 
Erupted,  Seventeenth  to  Twentieth  Year. 
Average  Length  of  Crown,  .24. 

Average  Length  of  Root,  .44. 

Average  Length  over  All,  .68. 

Calcification  of  this  tooth  takes  place  in  precisely  the  same  manner 
as  that  in  the  first  and  second  molar,  with  the  exception  of  the  number 
of  lobes,  which  are  sometimes  three  and  sometimes  four.  The  lime- 
salts  begin  to  accumulate  between  the  eighth  and  ninth  year,  and  con- 
tinue with  somewhat  more  activity  than  that  of  the  first  and  second  molar. 
Between  the  ninth  and  tenth  year  the  three  or  four  cusps,  of  which  the 
future  tooth  is  to  be  composed,  have  coalesced,  and  by  the  eleventh  year 
calcification  in  the  crown  of  the  tooth  is  completed;  at  the  end  of  the 
following  year  the  roots,  which  are  variable  in  number,  have  made 
considerable  progress;  at  the  fourteenth  year  they  are  calcified  to  about 
half  their  length,  while  at  a  period  between  the  eighteenth  and  nineteenth 
year  the  formative  process  is  completed  (Fig.  ioi).  This  tooth,  like 
the  cuspid,  is  usually  fully  formed  before  eruption  takes  place. 

This  tooth  is  subject  to  a  greater  variety  of  form  than  any  other; 
in  rare  instances  it  is  similar  in  general  outline  and  cusp  formation  to 
the  first  molar,  but  in  a  vast  majority  of  cases  it  is  dissimilar,  the  most 
constant  deviation  being  its  size,  which  on  the  average  is  about  one- 
third  less.  In  the  accompanying  illustration  (Fig.  102)  the  forms  most 
frequently  met  with  are  shown.  It  will  be  observed  that  the  contour 
of  the  tooth  in  general  is  much  more  rounded  than  either  the  first  or  second 
molar.     The  buccal  angles  of  the  crown  are  alone  well  marked,  the 


158 


ANATOMY 


mesial  and  distal  surfaces  passing  into  the  lingual  surface  by  a  long, 
gradual  sweep,  and  thus  obliterating  the  lingual  angles.  In  many 
instances  the  tooth  is  tritubercular,  and  is  usually  made  so  by  the  absence 
or  diminutive  size  of  the  distolingual  cusp.  Just  as  this  cusp  was  inclined 
to  degenerate  in  the  second  molar,  so  we  find  this  retrograde  develop- 
mental tendency  increased  in  the  third  molar.  With  this  change  in  the 
construction  of  the  occlusal  surface,  there  is  a  corresponding  variation 
in  the  grooves,  ridges,  and  fossae. 


Fig.  102. — Various  Types  of  Upper  Third  Molar. 


Mesial  Surface  of  the  Crown  (Fig.  103). — In  many  particulars 
this  surface  corresponds  in  form  and  outline  to  the  mesial  surface  of  the 
first  molar;  it  is,  however,  usually  much  more  convex,  seldom  presenting 
a  concavity  or  even  a  positive  flatness.  The  surface  is  not  only  rounded 
from  buccal  to  lingual,  but  also  from  the  cervical  line  to  its  occlusal 
margin.  Thus  formed,  a  point  of  contact  is  provided  near  the  center 
of  the  surface.  The  occlusal  margin,  the  buccal  margin,  and  the  cervical 
margin  are  almost  identical  to  those  of  the  first  molar,  but  in  most  in- 
stances the  lingual  margin  is  wanting,  the  surface  gradually  passing 
into  the  lingual  without  a  decided  line  of  demarcation. 

Distal  Surface  of  the  Crown  (Fig.  104).— This  surface  is  much 
less  extensive  in  comparison  to  the  size  of  the  crown  than  the  correspond- 
ing surface  of  either  the  first  or  second  molars.     It  is  decidedly  rounded 


UPPER    THIRD    MOLAR 


159 


Buccal 
Root 


Mesiobuc- 
cal  Cusp 


Mesial  Groove 
Fig.  103. — Upper  Third  Molar,  Mesial 
Surface. 


in  every  direction  and  is  frequently  crossed  by  the  distal  developmental 
groove,  and  sometimes  by  one  or  more  supplemental  grooves.  The 
general  form  of  the  surface  is  much  influenced  by  the  presence  or  absence 
of  the  distolingual  cusp;  with  the 
former,  the  surface  is  more  exten- 
sive, presenting  less  convexity  and 
resembling  more  closely  the  distal 
surface  of  the  first  and  second 
molars;  with  the  latter,  the  extent  Mesiobuc- 

'  '  cal  Root 

of  the  surface  is  decreased  and 
the  convexity  increased. 

Buccal  Surface  of  the 
Crown  (Fig.  105). — The  mesial 
portion  of  this  surface  is  in  no 
way  at  variance  with  the  mesial 
portion  of  the  buccal  surface  of 
the  first  or  second  molar,  but 
much  variety  of  form   exists   in 

the  distal  portion.  The  buccal  groove  which  serves  to  separate  these 
two  portions  is  located  well  toward  the  distal  third  of  the  surface,  thus 
reducing  the  size  of  the  distal  portion  to  about  one-third  that  of  the 
mesial  portion.     In  general,  the  surface  is  but  little  more  convex  than 

the  corresponding  surface  of  the 
first  and  second  molar.  Its  mesial 
border  is  definitely  outlined,  as 
are  also  the  cervical  and  occlusal 
margins,  but  the  distal  margin 
cannot  be  definitely  located,  the 
surface  tending  to  pass  gradually 
into  the  distal  surface.  Like  the 
distal,  the  extent  of  this  surface 
is  much  regulated  by  the  size  and 
shape  of  the  distobuccal  cusp, 
which,  like  the  distolingual  cusp, 
is  inclined  to  degenerate. 
Lingual  Surface  of  the  Crown  (Fig.  106). — Like  the  distal  surface 
previously  described,  the  form  of  this  surface  is  much  influenced  by  the 
presence  or  absence  of  the  distolingual  cusp.  When  this  cusp  is  wanting 
or  but  little  developed,  the  surface  presented  is  decidedly  convex  and 
smooth;  in  many  instances  the  mesiodistal  curvature  described  is  almost 


Buccal 
Groove 


Fig.  104.- 


Mesiolin- 
gual  Cusp 


-Upper  Third  Molar,  Distal 
Surface. 


i6o 


ANATOMY 


a  perfect  semicircle,  and  in  passing  from  the  cervical  line  to  the  occlusal 
margin  the  surface  is  carried  well  toward  the  center  of  the  crown  by 
a  long  gradual  sweep  toward  the  lingual.  The  lingual  groove  is  usually 
absent.     The  change  in  form  produced  by  the  presence  of  the  disto- 

lingual  cusp  is  principally 
noticeable  in  a  less  pro- 
nounced convexity  and  the 
presence  of  the  lingual 
groove,  which  may  be 
noticed  as  a  slight  depres- 
sion or  as  a  well-defined 
groove.  This  groove,  when 
present,  is  always  located 
near  what  would  represent 
the  distolingual  angle  of  the 


Mesiobuc- 
cal  Root 


Mesiobuc- 
cal  Cusp 


Lingual  Root 


Distobuccal 
Root 


Distal 
Groove 
Distal  buccal 
Cusp 


Fig.  105. — Upper  Third  Molar,  Buccal 
Surface. 


crown;  the  distolingual  cusp 
seldom  if  ever  being  of  suf- 
ficient size  to  force  its  location  near  the  center  of  the  surface  as  in  the 
first  molar.  In  some  instances  this  groove  is  shown  upon  the  lingual 
surface  when  the  cusp  is  not  present;  in  this  case  the  distomarginal 
ridge  represents  in  a  manner  the  cusp  by  its  bold,  heavy  development. 
Occlusal  Surface  of  the  Crown  (Fig.  107). — When  looking  directly 
upon  this  surface,  an  opportunity  is  presented 
to  study  the  general  contour  of  the  crown;  the 
most  noticeable  difference  in  this  respect  be- 
tween this  tooth  and  the  first  molar  being  ob- 
served in  its  smaller  size,  and  the  absence  of 
well-marked  angles.  It  will  be  noted  that  the 
mesial  and  buccal  outlines  in  a  measure  resem- 
ble the  corresponding  outlines  of  the  first  and 
second  molars,  but  there  is  scarcely  any  similarity 
existing  when  comparing  the  distal  and  lingual 
outlines.  In  some  instances  the  crown  is  tri- 
angular (Fig.  103) ;  in  others  the  mesiobuccal  and 
bucco-marginal  outlines  form  an  obtuse  angle, 
the  free  ends  of  which  are  joined  together  by  a 

long  semicircle,  the  latter  constituting  the  distal  and  lingual  outlines  (Fig. 
103).  Again,  almost  the  reverse  of  this  last-mentioned  form  is  seen,  the 
buccal  and  distal  outlines  constructing  the  angle,  while  the  semicircular 
connection  between  the  two  is  made  up  of  the  mesial  and  lingual  outlines. 


Fig.  106.  —  Upper 
Third  Molar,  Lingual 
Surface. 


UPPER    THIRD    MOLAR 


161 


The  Marginal  Ridges. — The  mesiomar  ginal  ridge  is  usually  well 
defined,  and  in  most  instances  is  crossed  near  its  center  by  the  mesial 
groove,  and  frequently  by  two  or  more  supplemental  grooves.  This 
marginal  ridge  is  probably  the  most  constant  in  form,  the  numerous 
variations  to  which  the  surface  is  liable  seldom  making  any  material 
alteration  in  it.  Unlike  the  ridge  above  described,  the  distomar ginal 
ridge  is  most  variable  in  its  construction,  nearly  all  of  the  forms  charac- 
teristic of  the  occlusal  surface  exerting  a  controlling  influence  over  it. 
In  rare  instances  the  ridge  resembles  that  of  the  first  and  second  molars, 
but  this  form  is  most  frequently  interfered  with  by  the  absence  or  diminu- 
tive size  of  the  distolingual  cusp,  the  ridge  itself  frequently  supplying  the 
place  of  the  cusp.  In  many 
cases  the  ridge  is  elevated  near 
its  central  part  by  being  rein- 
forced by  a  portion  of  the 
oblique  ridge.  When  the  dis- 
tolingual cusp  is  wanting,  this 
ridge  not  infrequently  de- 
scends from  the  summit  of 
the  distobuccal  cusp  to  the 
distal  groove  and  from  this 
point  ascends  obliquely  to  the 
summit  of  the  lingual  cusp. 

The  bucco -mar ginal  ridge  may  be  described  as  similar  in  most  respects  to 
the  corresponding  margin  on  the  first  and  second  molars,  the  principal  vari- 
ation being  in  the  distal  half,  which  is  much  shorter  and  less  pronounced. 
In  the  linguomar ginal  ridge,  again,  much  variety  in  outline  is  noticeable. 
In  nearly  all  instances  the  ridge  is  thrown  much  nearer  the  center  of  the 
body  of  the  crown,  and,  when  the  tooth  is  bicuspid  in  form,  it  simply 
makes  a  mesial  ascent  of  the  lingual  cusp,  followed  by  a  gradual  incline, 
and  passes  into  the  distal  ridge,  as  above  noted.  When  the  distolingual 
cusp  is  present,  the  ridge  is  similar  to  that  upon  the  first  and  second 
molars,  with  the  exception  of  the  distal  portion,  which  is  less  clearly 
marked. 

The  Cusps  (Fig.  107). — As  previously  stated,  the  form  most  frequently 
met  with  is  tritubercular,  two  of  the  cusps  being  upon  the  buccal  and 
one  upon  the  lingual  half  of  the  surface. 

The  Mesiobuccal  Cusp  (Fig.  107). — This  cusp  corresponds  in  nearly 
every  particular  to  the  mesiobuccal  cusp  of  the  first  and  second  molars; 
it  is  the  most  constant  in  size  and  form  of  the  three.     Its  summit  is  usually 


Fig.  107. — Upper  Third  Molar,  Occlusal 
Surface. 


1 62  ANATOMY 

angular,  and  the  numerous  ridges  which  descend  from  it  are  well  defined 
and  similar  in  name  and  number  to  those  of  the  first  molar. 

Distobuccal  Cusp  (Fig.  107). — The  constant  inclination  to  degeneracy 
in  the  distal  portion  of  the  crown  of  the  tooth  is  noticeable  in  this  cusp, 
which  is  much  smaller  than  the  mesiobuccal  and  scarcely  half  as  large 
as  the  corresponding  cusp  of  the  first  and  second  molars.  In  some 
instances,  however,  it  is  inferior  only  in  size,  retaining  its  angularity, 
being  possessed  of  small  but  well-defined  ridges. 

The  Lingual  Cusp  (Fig.  107). — When  the  three  cusps  alone  are  pres- 
ent, this  one  is  much  the  largest,  the  extent   of   the  surface  covered 

being  all  of  the  lingual  half  of  the  crown.  The 
summit  of  the  cusp,  which  is  thrown  well  toward 
the  center  of  the  body  of  the  crown,  is  prominent, 
but  seldom  angular.  Only  in  rare  instances  will 
there  be  found  a  lingual  ridge  descending  there- 
from, but  the  central  incline  is  usually  marked 
by  a  number  of  wrinkles  or  folds  of  enamel  re- 
sembling minute  ridges.  The  central  boundary 
of  this  cusp  is  marked  by  the  mesial  and  distal 
Fig.  108.— Upper  Third      developmental  grooves. 

Molar,    Occlusal    Surface,  .       _^ .       ,.  7    „  „.  .         T     .       , 

with    Distoiingual    Cusp  The  Distoiingual  Cusp  (Fig.  107).— It  is  the 

and  Distal  Fossa  Poorly      presence  or  absence  of  this  cusp  that  contributes 

Denned.  * 

most  to  the  variations  present  in  the  crown. 
When  present,  it  is  usually  diminutive  in  size,  and  is  without  definite 
form.  In  many  instances  nature  is  apparently  attempting  to  cast  it  off 
in  precisely  the  same  manner  in  which  she  is  attempting  to  add  to  the 
first  molar  by  a  development  of  the  "fifth  cusp,"  the  distobuccal  cusp 
appearing  to  hang  to  the  distoiingual  angle  of  the  crown  in  a  manner 
very  similar  to  the  "fifth  cusp."  When  thus  situated,  it  is  separated 
from  the  body  of  the  crown  by  a  groove,  which  cannot  be  considered  as 
being  upon  the  occlusal  surface.  When  located  in  its  normal  position, 
it  has  for  its  inner  boundary  the  distoiingual  groove. 

The  Fossae  and  Grooves  of  the  Occlusal  Surface. — The  great 
variety  and  form  common  to  this  surface  exerts  a  controlling  influence 
over  the  size,  number,  and  position  of  the  grooves  and  fossae.  In  the 
tritubercular  class  the  central  fossa  alone  is  present.  The  developmental 
grooves,  with  the  exception  of  the  buccal,  are  not  definitely  outlined, 
but,  descending  toward  the  fossae  from  the  central  incline,  are  numerous 
small  ridges  divided  from  each  other  by  a  like  number  of  diminutive 
supplemental  grooves.     The  distal  groove  is  sometimes  well  defined, 


UPPER    THIRD    MOLAR  1 63 

and  crosses  over  the  oblique  ridge,  which  in  this  type  becomes  the  disto- 
marginal  ridge.  When  the  distolingual  cusp  is  present,  all  of  the  ridges 
and  grooves  are  more  pronounced.  In  this  case  the  central  fossa  cor- 
responds more  closely  to  the  central  fossa  of  the  other  molar  teeth,  this 
resemblance  increasing  just  in  proportion  as  the  size  of  the  distolingual 
cusp  increases.  The  distal  fossa,  in  a  vast  majority  of  instances,  is 
present  as  a  mere  pit;  the  size  of  this  fossa  is  likewise  much  controlled 
by  the  extent  of  development  in  the  distolingual  cusp.  Where  the  disto- 
marginal  ridge  is  supplementary  to  the  distolingual  cusp,  the  distolingual 
groove  lies  between  the  former  and  oblique  ridge.  Another  peculiarity 
found  only  upon  the  occlusal  surface  of  this  tooth  is,  what  appears 
to  be  an  effort  upon  the  part  of  the  cusps  to  cluster  toward  the  center. 
This  is  common  only  to  those  teeth  possessing  three  cusps,  and  accompany- 
ing this  form  the  central  fossa  shows  a  number  of  fantastically  arranged 
grooves  and  ridges  which  ascend  the  cusps,  passing  over  the  marginal 
ridges  and  breaking  them  into  a  number  of  small  tubercles. 

Temperamental  Types. — The  third  molar  is  probably  less  influenced 
by  the  character  and  habits  of  the  individual  than  any  other  tooth  in 
the  mouth.  The  inclination  to  a  general  degeneracy  is  no  doubt  favored 
by  civilization.  With  a  constant  decline  in  the  functional  activity, 
brought  about  by  the  present  culinary  methods  common  to  civilization, 
this  tooth  in  a  measure  becomes  useless,  and  nature  is  gradually  mak- 
ing an  effort  to  cast  it  off.  While  there  are  undoubtedly  many  indi- 
viduals possessed  of  the  highest  mental  attainments  with  the  third 
molar  as  fully  developed  as  either  the  first  or  the  second,  this  condition 
is  usually  confined  to  those  possessed  of  little  intellectuality.  If,  in 
general,  the  temperament  of  the  subject  be  taken  into  consideration, 
the  cusp-formation  on  this  tooth  will  correspond  in  a  relative  degree 
to  that  on  the  bicuspids  and  molars. 


CHAPTER  IX. 

A  Description  of  the  Lower  Teeth  in  Detail. — Calcification,  Erup- 
tion and  Average  Measurements. — Their  Surfaces,  Ridges, 
Fossae,  Grooves,  Sulci,  etc. 

THE  LOWER  TEETH. 

In  most  respects  the  anatomy  of  the  lower  teeth  is  similar  to  that 
of  the  upper,  but  in  each  class  we  find  a  slight  variation  existing  between 
the  two  sets.  As  compared  to  the  upper  incisors,  the  crowns  of  the 
lower  incisors  are  more  slender  and  somewhat  more  angular  in  outline. 
The  roots  are  more  slender,  proportionately  longer,  more  flattened 
laterally,  and  seldom  crooked.  The  crowns  of  the  lower  incisors  are 
probably  more  constant  in  form  than  those  of  any  other  teeth,  seldom 
varying  except  in  size.  The  mesiodistal  measurement  of  the  crown 
of  the  lateral  incisor  is  a  trifle  greater  than  that  of  the  central,  a  con- 
dition exactly  the  reverse  to  that  of  the  upper  incisors.  The  labial 
and  the  lingual  surfaces  of  these  teeth  are  smooth,  and,  with  the  exception 
of  young  teeth,  show  but  little  trace  of  the  developmental  process  by 
the  presence  of  grooves,  fissures,  etc.  The  outline  of  the  lower  cuspids 
is  almost  identical  to  that  of  the  corresponding  teeth  in  the  superior  arch, 
excepting  that  they  are  in  every  way  more  slender.  The  bicuspids  are 
proportionately  smaller  in  every  direction  than  those  of  the  upper  jaw, 
their  cusps  are  smaller,  and  they  are  seldom  found  with  more  than  one 
root.  The  crowns  of  the  lower  molars  are  somewhat  larger  than  those 
of  the  upper,  and  are  provided  with  five  cusps  instead  of  four,*  and  they 
are  attached  to  the  alveolus  with  two,  instead  of  three,  roots.  In  the 
incisors,  cuspids,  and  bicuspids  the  process  of  development  is  the  same 
as  in  the  corresponding  upper  teeth,  calcification  taking  place  from  the 
same  number  of  centers  along  the  coronal  extremities.  In  the  molars, 
however,  development  may  proceed  from  five  centers  instead  of  four, 
as  in  the  upper  molars.  The  manner  of  development,  and  the  period 
at  which  this  action  takes  place,  so  nearly  corresponds  with  that  of  the 
upper  teeth  that  the  process  will  not  be  repeated. 

*  The  lower  first  molar  has  five  cusps  in  ninety  per  cent,  of  cases,  while  in  the  second 
five  cusps  are  present  in  about  fifty  per  cent. 

164 


LOWER    CENTRAL    INCISOR  1 65 


LOWER  CENTRAL  INCISOR. 

Calcification  Begins,  First  Year  after  Birth. 

Calcification  Completed,  about  the  Tenth  Year. 
Erupted,  Seventh  to  Eighth  Year. 
Average  Length  of  Crown,  .34. 

Average  Length  of  Root,  .47. 

Average  Length  over  All,  .81. 

Like  the  upper  central  incisor,  this  tooth  presents  for  examination 
four  surfaces,  a  cutting-edge,  and  various  angles,  margins,  etc.  By  the 
union  of  the  labial  and  lingual  surfaces  at  the  cutting-edge  the  incisive 
feature  is  established  and  the  double  incline  plane  common  to  incisors 
produced. 

The  Labial  Surface  of  the  Crown  (Fig.  109). — This  surface  is 
smooth  and  convex,  its  general  outline  resembling  an  inverted  cone, 
the  base  of  which  is  formed  by  the  cutting-edge  and  the  apex  by  the 
cervical  line.  The  margins  of  the  surface 
are,  with  the  exception  of  the  cutting-edge, 
not  so  well  defined  as  those  of  the  upper 
central.  Near  the  cutting-edge  the  mesial 
and  distal  margins  pass  somewhat  abruptly 
into  the  respective  lateral  surfaces,  but  as 
the  neck  of  the  tooth  is  approached  they  are 
much  rounded.  The  incisive  margin 
squarely  cut,  and  is  nearly  at  right  angles 
with  the  long  axis  of  the  tooth.  The  cervical 
margin  is  fairly  well  defined,  and  is  deeply 
concave  in  the  direction  of  the  root.  Except 
in  very  young  teeth,  this  surface  is  seldom 
much  broken  by  the  labial  grooves;  but  in 
certain  tvpes  one  or  more  transverse  ridges 

.  .  .....  Fig.  109. — Lower  Incisor,  Right 

may  be  found  occupying  the  cervical  third.  side,  Labial  Surface. 

The  mesiodista!  diameter  at  the  cutting- 
edge  is  about  one-third  greater  than  at  the  cervical  line,  and,  while 
these  measurements  are  likely  to  vary  in  accordance  with  the  tempera- 
ment of  the  subject,  this  variation  is  not  so  pronounced  as  in  the  upper 
incisor.  The  surface  is  frequently  inclined  to  flatness  near  the  incisive 
margin,  the  general  convexity  becoming  more  marked  as  the  cervical 
line  is  approached. 

The  Lingual  Surface  of  the  Crown  (Fig.  no). — In  general  out- 
line this  surface  resembles  the  labial,  with  the  exception  of  the  cervical 


1 66 


ANATOMY 


Lingual  Grooves 


Mesiomar- 
ginal  Ridge 


Lingual 
Fossa 


Fig.  iio. — Lower  Incisor,  Right  Side, 
Lingual  Surface,  Strongly  Developed. 


margin,  the  lines  of  which  are  somewhat  more  acute.  The  surface 
presents  a  marked  concavity  from  the  cutting-edge  to  the  cervical  ridge, 
and  also  a  slight  transverse  concavity  near  the  incisive  margin.     All  of 

the  margins  are  more  definite  than 
those  of  the  labial  surface.  The 
mesial  and  distal  margins  are  formed 
by  the  marginal  ridges  common  to 
these  borders,  but  these  ridges  are  not 
so  well  defined  as  those  of  the  upper 
incisors.  The  cervicomarginal  ridge 
is  present  as  a  well-rounded  band  of 
enamel,  but  is  never  a  well-defined 
cingulum,  or  cuspule.  The  depres- 
sion between  these  marginal  ridges  is 
so  slight  that  it  can  scarcely  be  re- 
ferred to  as  a  fossa,  although  usually 
characterized  as  the  lingual  fossa. 
The  lingual  grooves  are  generally  more  pronounced  than  the  corres- 
ponding developmental  grooves  of  the  labial  surface,  but  end  more 
or  less  abruptly  before  reaching  the  cervical  ridge.  The  mesiodistal 
measurements  of  the  surface  are  a  trifle  less  than  the  corresponding 
measurements  of  the  labial  surface. 

The  Mesial  Surface  of  the  Crown  (Fig. 
in). — The  outline  of  this  surface  is  exactly 
the  reverse  of  the  labial  and  lingual  just  de- 
scribed, being  a  cone,  with  its  base  directed 
downward  or  in  the  direction  of  the  root, 
while  its  apex  is  formed  by  the  mesial  ex- 
tremity of  the  cutting-edge.  The  cervical 
margin  of  the  surface,  or  that  represented  by 
the  base  of  the  cone,  is  concave;  the  labial 
and  lingual  margins  are  rounded  over  the 
cervical  third  and  inclined  to  angularity  near 
the  cutting-edge.  There  is  a  slight  convexity 
over  the  entire  surface,  which  is  most  marked 
near  the  center.  The  lingual  half  of  the 
cervical  portion  slopes  away  to  the  distal, 
passing  gradually  into  the  lingual  surface.  By  the  union  of  this  surface 
with  the  cutting-edge  and  the  labial  and  lingual  surfaces,  the  mesial 
angle  of  the  crown  is  formed.     This  angle  is  well  outlined  and  reaches 


Cutting-edge 


Cervical 
Line 

Cervical 
Ridge 


LOWER    CENTRAL   INCISOR 


167 


Labial 
Ridge 

Cervical 
Line 


Fig.  112.- 


-Lower  Incisor,  Distal 
Surface. 


out  toward  the  median  line,  giving  to  this  portion  of  the  crown  a  promi- 
nent appearance.  Near  the  cutting-edge  the  surface  presents  a  slightly 
rounded  prominence,  which  provides  a 
point  of  contact  with  the  corresponding 
tooth  of  the  opposite  side. 

The  Distal  Surface  of  the  Crown 
(Fig.  112). — In  a  general  way  this  sur- 
face closely  resembles  that  of  the  opposite 
or  mesial  side  of  the  crown.  Near  the 
cutting-edge  the  surface  is  usually  more 
prominent  and  presents  a  more  marked 
convexity,  and  near  the  cervical  margin 
it  is  flattened  and  sometimes  slightly  con- 
cave. The  union  of  this  surface  with  the 
cutting-edge  and  the  labial  and  lingual 
surfaces  forms  the  distal  angle  of  the 
crown,  which,  like  the  mesial  angle,  is 
square  and  well  defined.  The  margins  of  the  surface  are  in  no  way 
different  from  those  of  the  mesial  surface. 

The   Cutting-edge. — In   the   young   tooth   this   incisive   margin   is 
_     ,  ,  „  thin  and  generally  divided  into  three  distinct  parts 

Developmental    Grooves  °  J  x 

(Fig.  113)  by  the  developmental  grooves,  but  these 
disappear  so  early  that  they  can  scarcely  be  considered 
in  connection  with  a  description  of  the  fully  developed 
tooth;  in  fact,  the  cutting-edge  of  this,  as  well  as  that 
of  all  the  lower  incisors,  is  so  susceptible  to  change  by 
mechanical  abrasion  that  a  normal  condition  is  of  but 
short  duration.  After  the  disappearance  of  the 
primitive  cusps,  and  before  further  abrasion  has  taken 
place,  the  edge  is  fairly  sharp  and  placed  nearly  at 
right  angles  with  the  crown.  As  in  the  upper  incisors, 
the  cutting-edge  is  in  a  line  with  the  long  axis  of  the 
tooth.  The  labial  margin  of  the  edge  is  slightly  con- 
vex, while  the  lingual  is  irregularly  concave  to  the 
same  extent. 

The  Cervical  Margin. — This  marginal  line, 
which  is  marked  by  the  extent  of  the  enamel  cap, 
corresponds  closely  to  that  of  the  upper  incisors, 
dipping  down  with  a  graceful  concavity  on  the  labial  and  lingual  surfaces 
with  a  corresponding  convexity  on  the  mesial  and  distal  surfaces.    The 


Fig.  113. — Young 
Lower  Incisor,  with 
Cutting-edge,  show- 
ing the  Lines  of 
Development. 


l68  ANATOMY 

prominence  of  this  enamel  margin,  together  with  the  nature  of  the 
curvature,  is  much  influenced  by  the  tooth  type. 

The  Neck  of  the  Tooth. — A  distinctive  feature  of  this  tooth  is 
found  in  the  convergence  of  its  mesial  and  distal  surfaces  in  passing 
from  the  cutting-edge  rootward,  thus  producing  a  neck  much  constricted 
from  mesial  to  distal.  When  examined  from  either  the  mesial  or  distal 
surface,  this  feature  is  scarcely  noted,  the  crown  passing  into  the  root 
with  little  more  than  the  cervical  line  as  a  mark  of  separation.  The 
labial  and  lingual  portions  of  the  neck  are  rounded  and  narrow,  while 
the  two  lateral  sides  are  flat  and  broad. 

The  Root. — The  root  of  this  tooth  is  usually  smaller  than  that  of 
any  other  tooth  in  the  mouth.  It  is  much  flattened  from  mesial  to  distal, 
while  the  labial  and  lingual  aspects  are  rounded  and  narrow.  Besides 
being  flattened  and  broad,  the  mesial  and  distal  sides  are  usually  found 
with  a  longitudinal  depression  extending  from  a  point  near  the  base  of 
the  root  almost  to  its  apex.  These  surfaces  gradually  taper  from  the 
base  to  the  apex,  while  the  labial  and  lingual  first  widen  from  the  base 
and  then  gradually  taper  to  the  apex.  The  contour  of  the  root-base 
is  generally  reduced  at  the  apical  extremity,  although  in  some  instances 
the  latter  is  a  rounded  point.  While  the  root  of  the  tooth  is  usually 
straight,  there  is  sometimes  a  tendency  for  the  apical  third  to  have  a 
slight  distal  inclination. 

LOWER  LATERAL  INCISOR. 

Calcification  Begins,  First  Year  after  Birth. 

Calcification  Completed,  Tenth  to  Eleventh  Year. 
Erupts,  Eighth  to  Ninth  Year. 
Average  Length  of  Root,  .35. 

Average  Length  of  Root,  .50. 

Average  Length  over  All,  .85. 

The  crown  of  this  tooth  differs  from  the  central  incisor  in  being 
broader  from  mesial  to  distal  at  the  cutting-edge,  resulting  in  a  crown 
more  strongly  bell-shaped.  The  cutting-edge,  instead  of  being  at  right 
angles  to  the  long  axis  of  the  tooth,  slopes  to  the  distal  at  the  expense 
of  the  distal  angle,  which  is  much  rounded,  while  the  mesial  angle  closely 
resembles  the  corresponding  angle  of  the  central  incisors.  The  labial 
and  mesial  surfaces  do  not  differ  materially  from  the  corresponding 
surfaces  of  the  central  incisor,  excepting  that  the  lingual  more  frequently 
shows  the  lines  of  development,  and  the  distal  is  at  variance  in  having 
that  portion  which  contributes  to  the  formation  of  the  distal  angle  ex- 
tended and  prominent.     The  marginal  ridges  of  the  lingual  surface  are 


LOWER    CUSPID 


169 


probably  more  definitely  outlined  than  those  of  the  central  incisor,  and 
the  crown  in  general  presents  a  stronger  appearance.  The  neck  of  the 
tooth  is  similar  to  the  neck  of  the  central  incisor,  as  is  also  the  root,  with 
the  exception  of  a  slight  addition  to  its  length. 


LOWER  CUSPID. 

Calcification  Begins,  Third  Yeah  after  Birth. 

Calcification  Completed,  Twelfth  to  Thirteenth  Year. 
Erupts,  Twelfth  to  Thirteenth  Year. 
Average  Length  of  Crowx,  .40. 

Average  Length  of  Root,  .60. 

Average  Length  over  All,  i.oo. 

There  is  probably  a  greater  similarity  existing  between  the  upper 
and  lower  cuspid  than  in  any  other  class  of  teeth  in  the  mouth.  Oc- 
cupying as  they  do  a  prominent  position  in  the  dental  arch,  and  being 
called  upon  to  perform  the  double  function  of  incising  and  tearing  the 
food,  their  crowns  are  strong  and  heavy-set,  and  their  roots  long  and 
firmly  anchored  in  the  alveoli.  Like  the  upper  cuspid,  the  crown  of  the 
lower  is   surmounted   by   a  single  Labial  Grooves 

cusp,  from  the  summit  of  which 
descend  a  mesial  and  a  distal 
cutting-edge.  There  is  also  a 
labial,  lingual,  mesial,  and  distal 
surface  presented  for  examination. 

The  Labial  Surface  of  the 
Crown  (Fig.  114). — The  crown  of 
the  tooth,  being  a  little  longer  than 
that  of  the  upper  cuspid,  gives  to 
this  surface  the  appearance  of  be- 
ing more  slender,  when  in  reality 
there  is  but  little  difference  in  the 
width  of  the  two  teeth.  This  sur- 
face is  smooth  and  convex,  and, 
while  the  labial  grooves  are  usually 


Fig.  114. — Lower  CuspiJ,  Labial  Surface. 


present,  they  are  not  so  marked  as  those  found  upon  the  corresponding 
upper  tooth.  A  pronounced  feature  of  the  surface  is  the  labial  ridge, 
which  extends  from  the  summit  of  the  cusp  to  the  cervical  line,  provid- 
ing additional  strength  to  the  crown.  Aside  from  this  ridge  and  the  labial 
grooves,  the  surface  is  occasionally  broken  by  one  or  more  transverse 
ridges  over  the  cervical  portion.  The  margins  of  the  surface  closely 
resemble  those  of  the  upper  cuspid,  the  incisive  and  mesial  being  definite 


170 


ANATOMY 


Lingual  Grooves 


in  character,  while  the  distal  is  made  equally  indefinite  by  the  passing 

of  the  labial  into  the  distal  surface  by  a  gentle  curve. 

The  Lingual  Surface  of  the 
Crown  (Fig.  115). — The  ridges  and 
grooves  of  this  surface  are  far  less 
bold  in  their  character  than  those 
of  the  upper  cuspid.  The  lingual 
ridge,    which    divides    the    surface 


Distal  Angle 

'FDistomar- 
ginal  Ridge 


Ridge 


Fig.  11=;. 


-Lower  Cuspid,  Lingual 
Surface. 


Lingual 
Ridge 
Mesiomar- 
ginal  Ridge 

cervical  into  two  equal  parts,  extends  from 
the  summit  of  the  cusp  to  the  base 
of  the  cervical  ridge,  while  the 
marginal  ridges  pass  rootward  from 
the  angles  of  the  crown,  and,  unit- 
ing, form  the  cervical  ridge.  The 
slight  depressions  between  the 
lingual  ridge  and  the  marginal 
ridges  correspond  to  the  palatal 
grooves  of  the  upper  cuspid,  but 
in  this  tooth  partake  more  of  the 

nature  of  fossae. 

The    Mesial    Surface   of  the   Crown    (Fig.    116). — A  peculiarity 

found  in  connection  with  this  surface  is  the  general  plane  existing  between 

the  crown  and  root-surface.     In  all 

other  teeth   the   mesial  and  distal 

surfaces  are  found  to  bulge  some- 
what   beyond    the    corresponding 

surface  of  the  root,  but  this  surface 

of   the   lower   cuspid   is   not   only 

usually  in  a  direct  line  with  the 

mesial  surface  of  the  root,  but  is 

occasionally  inclined  to  the  distal, 

resulting  in  a  crooked  or  bent  ap- 
pearance to  the  tooth.     In  addition 

to  this  individual  peculiarity,   the 

surface  is  flat  and  passes  by  a  long 

curve  to  meet  the  lingual  surface. 

The    Distal    Surface    of    the  Fig.  n6. 

Crown  (Fig.  117). — This  surface  is 

somewhat  less  in  extent  than  the  mesial  surface,  and,  in  place  of  being 

flat  and  in  line  with  the  root-surface,  that  portion  near  the  angle  of 


Labial 
Ridge 


Cervical 
Ridge 


Lingual 
Ridge 

Mesiomar- 
ginal  Ridge 


Cervical 
Line 


-Lower  Cuspid,  Mesial 
Surface. 


LOWER    CUSPID 


171 


Lingual 
Ridge 


Cervicolin- 
gual  Ridge 


Mesial 
Groove 


the  crown  presents  a  marked  convexity,  while  that  near  the  cervical  line 
is  frequently  slightly  concave.  This  general  form  of  the  surface  further 
assists  in  producing  the  distal  crook  previously  referred  to.  The  lingual 
margin  is  well  defined  and  somewhat  angular,  while  the  surface  passes 
so  gradually  into  the  labial  that  a  positive  line  of  demarcation  can  scarcely 
be  said  to  exist. 

The  Cusp  and  Cutting-edges. — In  most  respects  these  are  similar 
to  the  corresponding  parts  of  the  upper  cuspid.     The  length  of  the 
mesial  cutting-edge  is  usually  somewhat  less  than  that  of  the  distal,  but 
this  difference  is  seldom  so  marked 
as  that  found  in  the  upper  cuspid. 
The  mesial  and  distal  angles  of  the 
crown  are  equally  as  pronounced 
as  those  of  the  corresponding  upper 
tooth. 

The  Neck  of  the  Tooth  — 
This  is  shown  by  a  fairly  well- 
marked  constriction,  but  the  pass- 
ing of  the  mesial  surface  of  the 
crown  into  the  mesial  surface  of 
the  root  is  not  broken  by  this  cir- 
cular depression.  On  account  of 
this  latter  feature  the  neck  of  this 
tooth  is  somewhat  less  pronounced 
than  that  of  the  upper. 

The  Root  of  the  Lower  Cuspid. — The  root  of  this  tooth  is  some- 
what shorter  and  more  flattened  on  its  mesial  and  distal  sides  than  that 
of  the  upper  cuspid,  this  lateral  flatness  frequently  amounting  to  a 
decided  longitudinal  depression  or  groove.  As  referred  to  in  the  descrip- 
tion of  the  crown,  the  mesial  side  of  the  root  is  continuous  in  a  direct 
line  with  this  surface  of  the  crown,  but  as  the  apical  third  of  the  root  is 
approached,  there  is  frequently  found  a  slight  distal  inclination  which 
affects  alike  both  the  mesial  and  distal  sides.  The  labial  and  lingual 
surfaces  of  the  root  are  abruptly  convex  and  taper  very  gradually  from 
the  cervical  line  to  the  apex,  while  the  mesial  and  distal  surfaces  taper 
much  more  rapidly,  the  four  ending  in  a  slender  apex  usually  flattened 
from  mesial  to  distal. 


Fig.  117. — Lower  Cuspid,  Distal  Surface. 


172 


ANATOMY 


THE  LOWER  BICUSPIDS. 

In  many  respects  these  teeth  are  similar  to  the  bicuspids  of  the  up- 
per jaw,  the  chief  differences  being  that  they  are  somewhat  shorter  and 
smaller  in  every  respect.  Their  crowns  are  much  more  rounded  and 
the  cusps  are  never  so  strongly  developed.  Unlike  the  upper  bicuspids 
the  buccal  and  lingual  cusps  are  connected  by  a  transverse  ridge.  The 
roots  are  much  less  flattened  from  mesial  to  distal,  and  are  seldom 
bifurcated. 


Summit  of  Buccal  Cusp 


LOWER  FIRST  BICUSPID. 

Calcification  Begins,  about  the  Fourth  Year. 

Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupted,  Tenth  to  Eleventh  Year. 
Average  Length  of  Root,  .30. 

Average  Length  of  Crown,  .54. 

Average  Length  over  All,  .84. 

In  general,  the  crown  of  this  tooth  is  much  more  rounded  and 
smaller  in  all  its  measurements  than  that  of  the  upper  bicuspid.  The 
buccal  surface  presents  a  much  greater  convexity,  which  results  in 
forcing  the  summit  of  the  buccal  cusp  well  toward  the  center  of  the  long 
axis  of  the  tooth.     The  mesiodistal  and  buccolingual  measurements  of 

the  crown  are  nearly  equal,  and 
about   correspond   to   the  maxi- 
mum length  of  the  crown.     As 
Buccal  Groove  [n  the  Upper  bicuspids,  the  de- 

Mesial  Pit  rr  r 

velopment  of  this  tooth  is  similar 
to  that  of  the  incisors  and  cus- 
pids, the  buccal  cusp  being  de- 
rived from  three  lobes,  while 
the  lingual  results  from  a  single 
center. 

The  Occlusal  Surface  of  the  Crown  (Fig.  118).— This  surface 
is  so  unlike  that  of  the  corresponding  surface  of  the  upper  first  bicuspid 
that  a  separate  description  without  further  comparative  reference  is 
required.  In  general  outline  the  form  of  a  rounded  triangle  is  approached, 
the  buccal  margin  serving  as  one  side  of  the  triangle,  while,  by  the  union 
of  the  mesial  and  distal  margins  to  form  the  lingual,  the  remaining  sides 
are  established. 

The  Buccal  Cusp. — As  previously  stated,  the  summit  of  this  cusp 
is  thrown  well  toward  the  center  of  the  surface.     Descending  from  it 


Buccal  Ridge 


Distal  Pit 


Triangular 
Ridge 


Lingual  Ridge 
Fig.  118. — Lower  First  Bicuspid, 
Occlusal  Surface. 


THE    LOWER    BICUSPIDS 


173 


Buccal  Ridge 

Distobuccal 
Angle 

■     1 

Buccal 
Groove 

Cervical 
Ridge 

■     J 

Cervical 
Line 

Fig. 


119. — Right    Lower   First 
cuspid,  Buccal  Surface. 


Bi- 


are  four  well-defined  ridges — the  buccal  ridge  to  the  buccal  surface, 

the    mesial    cutting-edge,    the   distal    cutting-edge,    and    the    triangular 

ridge,  the  latter  descending  in  a  lingual 

direction  to  meet  the  lingual  ridge  or 

cusp.     This    ridge   divides    the   surface 

into  two  parts,  the  center  of  each  being 

marked  by  a  well-defined  pit — the  mesial 

and  distal  pits.     The  mesial  and    distal 

cutting-edges  are  frequently  crossed  by 

the  buccal  grooves,  and  mark  the  line 

of  union  between  the  central  and  two 

lateral  lobes  of  the  buccal  cusp.     The 

marginal  ridges,  one  of  which  begins  at 

the  mesial  angle  and  the  other  at  the 

distal  angle,  pass  to  the  lingual,  where 

they  unite  to  form  the  lingual  ridge  or 

cusp. 

The   Lingual    Cusp. — This    cusp    is 

seldom  well  developed,  and  corresponds  to  the  cervical  ridge  of  the 

incisors  and  cuspids.     The  extent  of  development  in  the  lobe  is  ex- 
tremely variable,  in  some  instances  amounting  to  little  more  than  a 

continuation  of  the  mesio-  and  disto- 
marginal  ridges,  while  in  others  there  is 
a  building-up  of  the  enamel  in  the  form 
of  a  small  tubercle.  When  this  latter 
condition  is  present,  the  triangular  ridge 
of  the  buccal  cusp  contributes  to  its 
formation.  The  triangular  ridge  fre- 
quently divides  into  two  or  more 
smaller  ridges,  which  usually  end  in  the 
mesial  pit,  but  in  some  instances  they 
continue  to  the  lingual,  and  divide  the 
lingual  ridge  into  two  or  more  smaller 
tubercles. 

The  Buccal  Surface  of  the  Crown 
(Fig.  119). — This  surface  is  smooth  and 


Buccal  Cusp  Triangular 
Ridge 


Distal  Pit 


Fig.  120. — Left  Lower  First  Bi- 
cuspid, Lingual  Surface. 


convex  in  all  directions,  and  in  general 
outline  there  is  but  little  variation  be- 
tween it  and  the  corresponding  surface  of  the  upper  first  bicuspid.  It 
is    traversed    from    the    point    of    the  cusp  to  the  cervical  line  with  a 


174 


ANATOMY 


Lingual 

Cusp  or 

Ridge 


Buccal 
Ridge 


Fig. 


121. — Left  Lower  First  Bicus- 
pid, Mesial  Surface. 


rounded   ridge,  the  buccal   ridge,  upon   either  side  of  which  are  the 
buccal  grooves. 

Lingual    Surface  of  the  Crown  (Fig.  120). — This  surface  is  more 

or  less  extensive  in  accordance  with 
the  character  of  the  lingual  lobe.  In 
most  instances  the  measurement  from 
the  summit  of  the  cusp  or  ridge  to  the 
cervical  line  is  about  one-half  that  of 
the  same  measurements  on  the  buccal 
surface.  From  mesial  to  distal  a  well- 
rounded  convexity  is  present,  while 
from  the  occlusal  margin  to  the 
cervical  line  it  is  straight  or  only 
slightly  convex.  The  surface  passes 
so  gradually  into  the  mesial  and  distal 
surfaces  that  no  definite  lateral  mar- 
gins exist. 

The  Mesial  Surface  of  the 
Crown  (Fig.  121). — In  the  region  of 
the  occlusal  margin  this  surface  is  prominent,  with  a  marked  convexity 
from  buccal  to  lingual;  but  as  the  cervical  margin  is  approached,  the 
surface  recedes  to  the  distal,  and  is 
flattened  or  is  possessed  of  a  slight 
general  convexity.  The  occlusal  and 
cervical  margins  alone  are  well  de- 
fined, the  buccal  being  gracefully 
rounded,  while  the  surface  passes  to 
the  lingual  with  a  long  curve. 

The  Distal  Surface  of  the 
Crown  (Fig.  122). — There  is  but 
little  difference  between  this  and  the 
mesial  surface;  the  occlusal  portion  of 
the  surface  is  somewhat  less  promi- 
nent, resulting  in  less  of  the  bell- 
shaped  appearance  to  this  side  of  the 
crown. 

The  Neck  of  the  Tooth.— The 
neck  of  this  tooth  is  marked  by  a  well-defined  constriction,  the  enamel 
of  the  crown  suddenly  folding  in  to  meet  the  cementum  of  the  root  at 
the  cervical  line,  forming  a  band  or  ridge  which  completely  encircles  the 


Triangular  Ridge 


Distal  Pit 


Lingual 
Ridge 


Cervical 
Ridge 


Fig, 


122. — Right  Lower  First  Bi- 
cuspid, Distal  Surface. 


THE    LOWER    BICUSPIDS 


J75 


tooth.  The  amount  of  constriction  appears  to  be  evenly  distributed 
between  the  various  parts,  so  that,  viewed  in  all  directions,  the  neck 
becomes  a  distinctive  feature  of  the  tooth. 

The  Root  of  the  Lower  First  Bicuspid.— The  root  of  this  tooth 
is  usually  straight  and  tapers  gradually  from  base  to  apex.  In  rare 
instances  it  is  bifurcated,  and  when  thus  formed,  those  portions  beyond 
the  point  of  separation  are  more  or  less  crooked.  In  the  single  root  the 
apical  third  often  curves  slightly  to  the  distal.  The  buccal  and  lingual 
sides  are  convex  throughout  their  entire  length,  while  the  mesial  and 
distal  may  be  slightly  convex,  flattened,  or  provided  with  a  slight  longi- 
tudinal concavity.  In  passing  from  buccal  to  lingual  the  mesial  and 
distal  sides  converge,  thus  resulting  in  a  narrowing  of  the  lingual  side 
of  the  root. 


Buccal  Cusp 


LOWER  SECOND  BICUSPID. 

Calcification  Begins,  Between  the  Fourth  and  Fifth  Year. 
Calcification  Completed,  Eleventh  to  Twelfth  Year. 
Erupts,  Eleventh  to  Twelfth  Year. 
Average  Length  of  Crown,  .31. 

Average  Length  of  Root,  .56. 

Average  Length  Over  All,  .87. 

In  general  contour  this  tooth  is  similar  to  the  lower  first  bicuspid, 
excepting  that  the  crown  is  somewhat  more  rounded  and  the  lingual 
cusp  more  fully  developed,  this  latter  feature  causing  it  to  closely  resemble 
the  upper  bicuspids.  The  crown  is  frequently  a  trifle  shorter  than  that 
of  the  lower  first  bicuspid,  but  the  length 
of  the  root  generally  exceeds  that  of  the 
latter,  making  this  the  longer  tooth  ofTriangular 
the  two.  Rldge 

The  Occlusal  Surface  of  the 
Crown  (Fig.  123). — The  occlusal  sur-  Distal  Pit 
face  of  this  tooth  presents  a  greater 
variety  in  form  than  any  other  tooth  of 
its  class.  The  general  outline  of  the 
surface  is  that  of  a  broken  circle,  in 
most  instances  the  mesial  and  distal 
margins  showing  almost  as  much  of  a 
convexity  as  that  of  the  buccal  and  lingual.  The  summit  of  the 
buccal  cusp  usually  extends  well  toward  the  center  of  the  surface, 
but  it  is  sometimes  forced  toward  the  buccal  by  an  increased  develop- 
ment in  the  lingual  cusp.     The  buccal  grooves,  which  cross  the  mesial 


Lingual  Cusp 

Fig.  123. — Lower  Second  Bicuspid, 
Occlusal  Surface. 


176 


ANATOMY 


as  those 

of  the   first   bicuspid 

Buccal  Ridge 

Distobuccal 
Angle 

H      M 

Groove 

Cervical 
Ridge 

m      jm 

Cervical 
Line 

Fig. 


124. — Right  Lower  Second  Bi- 
cuspid, Buccal  Surface. 


and  distal  cutting-edges  of  the  buccal  cusp,  are  seldom  so  well  defined 

but  they  occasionally  pass  over  these 
marginal  ridges  and  form  well-marked 
grooves,  which  end  in  the  mesial  and 
distal  pits.  The  triangular  ridge  of  the 
buccal  cusp  is  usually  more  prominent 
than  in  the  first  bicuspid,  and  divides 
the  surface  into  two  portions,  which  are 
about  equal  in  extent,  the  center  of  each 
portion  being  provided  with  a  small  pit 
— the  mesial  and  distal  pits.  As  in  the 
first  bicuspid,  the  mesio-  and  disto- 
marginal  ridges  begin  at  the  mesial  and 
distal  angles  of  the  crown,  pass  to  the 
lingual,  and,  uniting,  form  the  lingual 
ridge  or  cusp.  The  lingual  cusp,  while 
generally  well  developed,  is  never  so 
prominent  as  the  buccal.     The  lingual 

lobe  is  sometimes  divided  by  a  groove  which  passes  from  buccal  to 

lingual,  thus  forming  three  cusps  upon  the  surface.     When  this  latter 

condition  is  present,  the  mesial  and  distal  grooves  are  fully  outlined 

from    the    mesio-     and    disto-marginal  Buccal  Grooves 

ridges  to  the  center  of  the  surface,  where  ^^^^^m- 

they   unite   with   the   groove   previously 

referred  to   and   form   a  central   pit  or  °^f  Rfj£; 

fossa.     Another    form    frequently    met 

with  is  one  in  which  the  surface  closely 

resembles  that  of  the  upper  bicuspids, 

two   well-defined    cusps   being    present, 

separated  from  each  other  by  a  central 

groove,    which    passes    from    mesial    to 

distal  and  joins  the  triangular  grooves 

at  these  joints.     The  resemblance  to  the 

upper  bicuspids  is  further  increased  by 

the  presence  of  two  small  pits,  one  on 

the  mesial  and  one  on  the  distal  half  of 

the  surface. 

The  Buccal   Surface   of   the   Crown    (Fig.    124).— The  principal 

variation  between  this  and  the  buccal  surface  of  the  lower  first  bicuspid 

is  that  it  is  less  extensive  and  the  buccal  grooves  somewhat  less  defined. 


Summit 
of  Lin- 
gual Cusp 


Fig.  125. — Lower  Second  Bicuspid, 
Lingual  Surface. 


THE    LOWER    BISUCPIDS 


177 


Lingual  Cusp 


Buccal  Cusp 
Buccal  Ridge 


FlG.  126. 


— Lower  Second  Bicuspid, 
Mesial  Surface. 


It  presents  a  general  convexity,  which  is  most  pronounced  near  the 
center,  between  which  point  and  the  occlusal  margins  it  is  slightly  inclined 
to  flatness.  The  summit  of  the  buccal  cusp  is  usually  to  the  mesial  of 
the  center  of  the  occlusal  mar-  Triangular  Ridge 

gin,  so  that  the  mesial  cutting- 
edge  is  considerably  longer  than 
the  distal,  this  fact  also  resulting 
in  forcing  the  buccal  ridge  to  the 
mesial  of  the  center  of  the  sur- 
face. The  mesial  angle  of  the 
crown,  as  observed  when  looking 
directly  upon  the  buccal  surface, 
is  in  a  direct  line  with  the  mesial 
side  of  the  root,  while  the  distal 
angle  extends  beyond  this  cor- 
responding line,  and  gives  a 
prominent  or  bulging  appearance 
to  this  section  of  the  crown. 

The  Lingual  Surface  of  the  Crown  (Fig.  125). — Proportionately, 
this  surface  is  more  extensive  than  the  corresponding  surface  of  the  first 
bicuspid,  this  increase  being  produced  by  the  additional  development 
of  the  lingual  cusp.     It  is  well  rounded  from  mesial  to  distal,  and  passes 

into  these  surfaces  without  the  existence 
of  a  positive  line  of  separation.  From 
the  cervical  line  to  the  occlusal  margin  a 
slight  convexity  is  present.  The  general 
outline  of  the  surface  is  much  influenced 
by  the  conditions  present  upon  the  oc- 
clusal surface. 

The   Mesial  Surface  of  the  Crown 
(Fig.  126). — In  the  region  of  the  occlusal 
margins  this  surface  is  decidedly  convex 
from  buccal  to  lingual,  but  in  passing 
toward  the  cervical  line  a  gradual  flat- 
ness is  apparent,  which,  however,  seldom 
amounts    to    a    perfect    plane.     While 
there  is  a  gradual  convergence  of  this 
and  the  distal  surface  toward  the  root,  it  is  not  so  marked  as  that  of 
the  first  bicuspid,  resulting  in  less  of  the  bell-shaped  appearance  to 
the  crown. 


Buccal  Groove 


Lingual 
Cusp 


FlG.    127. 


— Lower  Second  Bicuspid, 
Distal  Surface. 


178  ANATOMY 

The  Distal  Surface  of  the  Crown  (Fig.  127). — The  description 
given  of  the  mesial  surface  applies  equally  well  to  this,  there  being  but 
slight  variation  existing  between  the  two.  Occasionally  this  surface 
will  present  a  greater  convexity  in  the  region  of  the  occlusal  margin,  but 
this  is  not  a  constant  feature. 

The  Neck  of  the  Tooth. — The  crown  of  the  tooth  being  some- 
what smaller,  and  the  root  proportionately  larger  and  longer  than  that 
of  the  first  bicuspid,  results  in  diminishing  the  amount  of  constriction 
at  the  neck,  and  for  that  reason  this  feature  is  less  definite. 

The  Root  of  the  Tooth. — As  previously  stated,  the  root  of  this 
tooth  is  larger  and  longer  than  that  of  the  first  bicuspid.  The  mesial 
and  distal  sides  are  flattened  and  frequently  provided  with  a  longitudinal 
groove.  In  some  instances  it  is  rather  blunt,  ending  in  a  heavy,  rounded 
apex;  in  others  it  tapers  very  gradually  from  the  base  to  the  apex,  ending 
in  a  slim,  pointed  extremity. 

THE  LOWER  MOLARS. 

THE  LOWER  FIRST  MOLAR. 

Calcification  Begins,  about  One  Month  before  Birth. 
Calcification  Completed,  Ninth  to  Tenth  Year. 
Erupts,  Sixth  to  Seventh  Year. 
Average  Length  of  Crown,  .30. 

Average  Length  of  Root,  .52. 

Average  Length  over  All,  .82. 

The  process  of  development  in  this  tooth  corresponds  to  that  of 
the  upper  first  molar,  calcification  beginning  upon  the  various  cusps 
as  early  as  the  eighth  fetal  month,  the  crown  being  completely  calcified 
by  the  fifth  year,  the  roots  formed  and  the  root  apices  established  by 
the  eleventh  year.  There  is,  however,  one  important  difference  between 
the  development  of  this  tooth  and  the  corresponding  upper  tooth;  that 
of  calcification  taking  place  usually  from  five  centers  instead  of  four,  and, 
as  a  result,  we  find  the  occlusal  surface  provided  with  five  well-developed 
cusps  separated  from  one  another  by  five  developmental  grooves.  When 
compared  with  the  upper  first  molar,  the  crown  of  this  tooth  is  found  to 
be  somewhat  less  in  size;  in  general  outline  it  is  subject  to  a  greater  varia- 
tion and  is  much  more  angular  in  its  nature.  The  mediodistal  measure- 
ment of  the  crown  is  nearly  always  greater  than  the  buccolingual,  and 
the  length  of  the  crown  from  the  occlusal  margins  to  the  cervical  line  is 
proportionately  less  than  that  of  the  corresponding  upper  molar. 

The  Occlusal  Surface  of  the  Crown  (Fig.  128). — The  general 
outline  of  the  crown  is  best  studied  when  looking  directly  upon  this 


THE    LOWER    MOLARS 


179 


Buccal 
Cusp 


Distobuc- 
cal  Cusp 


Distal 
Groove 


surface.  Two  principal  varieties  exist:  one  in  which  the  sides  or  mar- 
gins of  the  surface  appear  to  be  flattened  or  straightened  out,  and  the 
other  when  these  same  margins  are  gracefully  rounded.  In  either 
form  the  buccal  line  is  the  longest,  so  that  the  mesial  and  distal  lines 
converge  to  meet  the  lingual.  This  common  form  gives  to  the  buccal 
angles  an  acute  character,  while  the  lingual  angles  are  about  equally 
obtuse.  The  surface  is  divided  into  five  distinct  or  developmental 
portions,  each  of  which  is  surmounted  by  a  cusp,  named,  as  their  location 
indicates,  mesiobuccal,  buccal,  distobuccal,  mesiolingual,  and  disto- 
lingual.  Separating  these  parts  are 
five  developmental  grooves — the 
mesial,  the  distal,  the  buccal,  the 
lingual,  and  the  distobuccal.  The 
four  former  cross  the  marginal 
ridges  from  the  various  surfaces 
and  end  in  the  central  fossa,  while  DlstalPltj 
the  latter  passes  from  the  disto- 
buccal angle  and  joins  the  distal 
groove,  their  union  being  marked 
by  a  slight  depression  or  pit — the 
distal  pit.  Branching  off  from  the 
various  grooves  are  a  number  of  supplemental  grooves,  the  presence  of 
which  results  in  the  production  of  a  number  of  smaller  ridges. 

The  Marginal  Ridges  of  the  Occlusal  Surface. — Properly  speaking 
these  are  only  two  in  number,  the  mesiomarginal  ridge  and  the  disto- 
marginal  ridge.  Those  margins  which  correspond  to  the  buccal  and 
lingual  ridges  of  the  upper  molars  are  so  broken  by  the  various  cusps 
and  developmental  grooves  that  a  definite  marginal  ridge  scarcely  exists, 
as  will  be  observed  by  the  description  of  these  parts. 

The  mesiomarginal  ridge  is  strongly  outlined,  passing  from  the 
mesiobuccal  to  the  mesiolingual  angle  of  the  crown  in  the  form  of  a  bold 
angular  ridge.  In  some  instances  it  is  broken  near  the  center  by  the 
mesial  groove  passing  over  it  to  reach  the  mesial  surface,  in  others  being 
further  divided  by  numerous  small  supplemental  grooves. 

The  distomar ginal  ridge  is  much  shorter  and  less  decided  than  the 
mesial,  and  extends  from  the  distobuccal  to  the  distolingual  angle. 
In  nearly  every  instance  it  is  broken  by  the  distal  groove,  which  crosses 
it  to  reach  the  distal  surface. 

The  buccomar ginal  ridge  is  formed  by  the  various  ridges  which 
descend  in  a  mesial  or  distal  direction  from  the  three  buccal  cusps. 


Disto-    Lingual  Mesio- 
lingual Groove  lingual 
Cusp  Cusp 

Fig.  128. — Lower  First  Molar,    Occlusal 
Surface. 


l8o  ANATOMY 

Near  the  center  the  margin  is  broken  by  the  buccal  groove,  and  it  is  again 
broken  at  its  distal  third  by  the  distobuccal  groove,  both  of  which  pass 
over  it  to  reach  the  buccal  surface  of  the  crown.  This  is  much  the 
longest  margin  of  the  surface,  and  in  its  entirety  presents  a  gradual 
buccal  convexity. 

The  linguomarginal  ridge  is  principally  made  up  of  the  distal  incline 
from  the  mesial  cusp,  and  by  the  mesial  incline  from  the  distal  cusp. 
Near  the  center  it  is  broken  by  the  lingual  groove,  which  passes  over  it 
to  reach  the  lingual  surface.  This  margin,  unlike  the  buccal,  is  not 
always  convex,  but  in  many  instances  is  almost  a  straight  line,  extending 
from  the  mesial  to  the  distal  angle. 

The  Cusps  (Fig.  128).  The  Mesiobuccal  Cusp  (Fig.  128). — This 
is  usually  the  largest,  though  not  always  the  longest,  cusp  of  the  group. 
It  is  bounded  by  the  mesial  and  buccal  surfaces  and  by  the  mesial  and 
buccal  grooves,  which  together  form  the  mesiobuccal  triangular  groove. 
Descending  from  the  summit  of  this  cusp  to  the  distal  is  a  well-defined 
ridge — a  part  of  the  buccomarginal  ridge — while  in  a  mesial  and  lingual 
direction  the  descending  ridges  contribute  to  both  the  bucco-  and  mesio- 
marginal  ridges.  Descending  toward  the  center  of  the  surface  and 
ending  in  the  central  fossa  is  the  mesiobuccal  triangular  ridge. 

The  Buccal  Cusp  (Fig.  128). — This  cusp,  which  is  placed  a  little 
to  the  distal  of  the  center  of  the  buccal  surface,  is  separated  from  the 
mesiobuccal  cusp  by  the  buccal  groove,  and  from  the  distobuccal  cusp 
by  the  distobuccal  groove.  It  is  about  one-half  the  size  of  the  mesio- 
buccal cusp,  and  a  trifle  less  in  length.  Descending  from  it  are  two  ridges, 
one  in  a  mesial  and  one  in  a  distal  direction,  which  form  a  portion  of  the 
buccomarginal  ridge;  descending  to  the  buccal  surface  is  the  buccal 
ridge,  while  the  central  incline  gives  place  to  a  fourth  ridge — the  bucco- 
triangular  ridge. 

The  Distobuccal  Cusp  (Fig.  128). — This  cusp  is  much  the  smallest 
of  the  five,  and  is  located  at  the  distobuccal  portion  of  the  crown,  in 
some  instances  being  nearest  the  buccal  surface,  in  others  forced  to  the 
distal  by  an  increase  in  the  size  of  the  buccal  cusp.  It  is  separated  from 
the  buccal  cusp  by  the  distobuccal  groove,  and  from  the  distolihgual  cusp 
by  the  distal  groove.  The  ridges  which  descend  from  it  contribute  to 
both  the  bucco-  and  disto-marginal  ridges,  and  descending  toward  the 
distal  pit  is  the  distobuccal  triangular  ridge. 

The  Mesiolingual  Cusp  (Fig.  128). — This  cusp  is  second  in  size, 
and  frequently  the  longest  and  most  pointed.  It  has  for  its  boundaries 
the  mesial  and  lingual  surfaces,  and  the  mesial  and  lingual  grooves. 


THE    LOWER    MOLARS 


I8l 


The  ridge  which  descends  from  it  in  a  mesiobuccal  direction  assists  in 
forming  the  mesiomarginal  ridge,  while  that  which  passes  to  the  distal 
forms  a  part  of  the  linguomarginal  ridge.  In  the  direction  of  the  central 
fossa  a  pronounced  ridge  is  present — the  mesiolingual  triangular  ridge 
— which  is  often  supplemented  by  one  or  more  smaller  ridges  running  in 
the  same  direction. 

The  Distolingual  Cusp  (Fig.  128). — This  cusp  usually  occupies  the 
distolingual  portion  of  the  crown,  although  sometimes  being  forced  well 

toward    the    lingual    by    the  Buccal  Groove 

distobuccal  cusp.  It  is 
separated  from  the  mesio- 
lingual cusp  by  the  lingual 
groove,  and  from  the  dis- 
tobuccal by  the  distal 
groove.  Two  of  the  ridges 
which  descend  from  it  as- 
sist in  forming  the  linguo- 
and  disto-marginal  ridges, 
while  the  one  which  de- 
scends the  central  incline  is 
the  distolingual  triangular 
ridge.  The  central  inclines 
of  the  mesiobuccal,  buccal, 
mesiolingual,  and  disto- 
lingual cusps  contribute  to 

the  formation  of  the  central  fossa,  while  the  buccal,  distobuccal,  and 
distolingual  central  inclines  assist  in  forming  the  distal  pit  or  fossa. 

The  Buccal  Surface  of  the  Crown  (Fig.  129). — This  is  the  most 
extensive  of  the  lateral  surfaces  of  the  crown.  It  is  convex  from  mesial 
to  distal,  and  also  from  the  occlusal  margin  to  the  cervical  line.  The 
width  of  the  crown  from  the  mesial  to  the  distal  angle  is  always  somewhat 
greater  than  that  at  the  cervical  line,  the  difference  being  governed! by 
the  typal  form  of  the  tooth.  A  little  to  the  mesial  of  the  center  of  the 
surface  is  the  buccal  groove,  which,  after  crossing  the  buccomarginal 
ridge,  is  usually  quite  deep;  but  as  it  proceeds  in  the  direction  of  the 
root  it  gradually  disappears,  or  it  may  end  abruptly  in  a  well-defined  pit 
— the  buccal  pit.  The  distobuccal  groove  enters  the  surface  near  the 
distobuccal  angle,  and  gradually  becomes  less  pronounced  as  it  passes 
rootward.  It  is  seldom  so  well  defined  as  the  buccal  groove,  and  usually 
ends  when  about  half-way  to  the  cervical  line.     The  occlusal  margin 


Fig.  129. — Lower  First  Molar,  Buccal 
Surface. 


182 


ANATOMY 


Distoliiigual    Mesiolingual 
Cusp  Cusp 


Fig.  130. — Lower  First  Molar, 
Lingual  Surface. 


is  made  irregular  by  the  presence  of  the  three  buccal  cusps;  the  cervical 
margin  is  nearly  straight  from  mesial  to  distal,  and  is  surmounted  through- 
out by  a  strong  enamel  fold,  the  cervicobuccal  ridge.     The  mesial  margin 

is  longer  than  the  distal,  but  neither  of 
them  is  well  defined. 

The  Lingual  Surface  of  the  Crown 
(Fig.  130). — This  surface  is  smooth  and 
convex  in  every  direction.  It  is  gener- 
ally divided  into  two  portions,  a  mesial 
and  a  distal,  which  are  nearly  equal  in 
extent.  This  separation  is  formed  by 
the  lingual  groove,  which  is  sometimes 
deep  and  sulcate,  at  others  shallow,  and 
not  infrequently  entirely  wanting.  The 
surface  is  nearly  one-third  less  in  extent 
than  the  buccal,  the  convergence  of  the 
mesial  and  distal  surfaces  in  passing  to 
the  lingual  accounting  for  this  difference. 
The  occlusal  margin  is  formed  by  the 
double  incline  of  the  two  lingual  cusps;  the  cervical  margin  is  either 
straight  or  slightly  concave  in  the  direction  of  the  occlusal  surface, 
while  the  mesial  and  distal  margins  are  rounded  and  poorly  defined. 

The  Mesial  Surface  of  the  Crown  (Fig.  131). — This  surface  is 
inclined  to  flatness,  with  a  slight 
bulging  near  the  center,  which  marks 
the  point  of  contact  with  the  approxi-  Mesiobuc- 1 
mate  tooth.  It  is  usually  smooth,  and 
unbroken  by  developmental  or  other 
grooves,  although  the  mesial  groove 
occasionally  traverses  it  after  crossing 
the  marginal  ridge  from  the  occlusal 
surface.  Near  the  center  of  the  cer- 
vical third  a  slight  concavity  is  often 
present.  The  margins  of  the  surface 
are  somewhat  irregular,  the  occlusal 
margin  being  made  irregularly  concave 
by  the  ridges  which  descend  from  the 
two  mesial  cusps;  the  cervical  margin  is  slightly  concave  in  the  direction  of 
the  occlusal  surface,  and,  while  the  buccal  margin  inclines  to  the  lingual  as 
the  occlusal  surface  is  approached,  the  lingual  is  almost  perpendicular. 


Central  Fossa 


cal  Cusp 
Cervico- 
buccal 
Ridge 


Fig.  131.- 


-Lower  First  Molar,  Mesial 
Surface. 


THE    LOWER    MOLARS 


183 


Fig. 


The  Distal  Surface  of  the  Crown  (Fig.  132). — Unlike  the  mesial, 
this  surface  is  possessed  of  a  decided  convexity  in  every  direction.  It 
is  surmounted  by  a  portion  of  the  distobuccal  and  distolingual  cusps,, 
and  is  frequently  broken  by  the  distal  groove,  which  reaches  it  after 
crossing  the  marginal  ridge  from  the  occlusal  surface.  The  occlusal 
margin  is  irregularly  formed  of  the  marginal  ridges  which  descend  from 
the  distobuccal  and  distolingual 
cusps;  the  cervical  margin  is 
usually  straight,  while  the  buc- 
cal and  lingual  are  rounded  and 
indefinite. 

The  Neck  of  the  Tooth.— 
One  characteristic  feature  of 
this  tooth  is  the  greater  circum- 
ference of  the  crown  at  the  oc- 
clusal margin  over  that  at  the 
cervical  line,  giving  a  flaring 
appearance  to  the  crown,  and 
resulting  in  the  production  of  a 
neck  which  is  much  constricted. 
This  is  particularly  noticeable 
when  looking  upon  the  buccal  surface  of  the  tooth;  but  when  looking 
upon  the  mesial  or  the  distal  surface,  this  feature  is  not  so  pronounced, 
although  the  rather  heavy  fold  of  enamel  which  surmounts  the  cervical 
line  contributes  much  to  the  formation  of  the  neck  from  these  aspects. 

The  Roots  of  the  Tooth.  —The  roots  of  this  tooth  are  two  in  num- 
ber— one  of  which  is  placed  beneath  the  mesial,  and  the  other  beneath 
the  distal  half  of  the  crown — and  are  named  the  mesial  root  and  the 
distal  root.  The  fact  that  the  point  of  bifurcation  is  constantly  in  close 
proximity  to  the  neck  or  crown  of  the  tooth  is  a  sufficient  reason  for 
the  statement  that  two  roots  exist,  rather  than  a  single  root  with  two 
branches.  The  roots  are  both  much  flattened  from  mesial  to  distal, 
and  broad  at  the  base  from  buccal  to  lingual. 

The  mesial  root  is  usually  the  larger  and  longer  of  the  two.  x\fter 
leaving  its  base  it  generally  inclines  to  the  mesial,  but  beyond  the  center 
of  its  length  it  is  provided  with  a  distal  turn,  which  in  some  instances 
amounts  to  a  decided  crook.  The  center  of  the  mesial  side  is  occupied 
by  a  longitudinal  depression,  as  is  also  the  distal  side,  making  this  part 
of  the  root  thin,  giving  the  appearance  of  an  effort  to  bifurcate,  which 
condition  is  occasionally  present.     The  buccal  and  lingual  sides  of  the 


-Lower  First  Molar,  Distal 
Surface. 


1 84 


ANATOMY 


root  are  rounded  and  smooth,  and  taper  gradually  to  the  apex,  which 
is  somewhat  broadened  from  buccal  to  lingual. 

The  distal  root  is  usually  straight,  with  a  more  gradual  taper  through- 
out, ending  in  an  apical  extremity  more  pointed  than  that  of  the  mesial 
root.  A  longitudinal  depression  is  also  present  upon  both  the  mesial 
and  distal  sides,  but  is  never  so  pronounced  as  that  upon  the  mesial  root. 
The  buccal  and  lingual  sides  are  convex  and  smooth.  The  root  possesses 
little  or  no  inclination  to  bifurcate. 


LOWER  SECOND  MOLAR. 

Calcification  Begins,  about  the  Fifth  Year. 

Calcification  Completed,  Sixteenth  to  Seventeenth  Year. 
Erupts,  Twelfth  to  Sixteenih  Year. 
Average  Length  of  Crown,  .27. 

Average  Length  of  Root,  .50. 

Average  Length  over  All,  .78. 

This  molar  differs  in  so  many  particulars  from  the  lower  first  molar 
that  a  separate  description  will  be  called  for.  The  principal  variation 
is  usually  found  in  the  absence  of  the  fifth  lobe  or  cusp,*  resulting  in  the 
production  of  an  occlusal  surface  much  less  complicated. 

The  Occlusal  Surface  of  the  Crown  (Fig.  133). — When  the  crown 
is  studied  by  looking  directly  upon  this  surface,  the  variations  between 

this  and  the  first  molar  are 
readily  noted.  Four  equally 
proportioned  cusps  are  observed, 
separated  from  each  other  by 
four  developmental  grooves.  A 
single  pit  or  fossa  is  present,  the 
four  grooves  arising  from  this 
one  point.  In  general  outline 
two  principal  varieties  exist:  one 
in  which  the  opposite  sides  of 
the  crown  are  nearly  of  the  same 
length,  and  parallel  with  each 
other,  with  the  angles  rounded; 
the  other,  in  which  either  the 
buccal  or  lingual  margin  is  the  longest,  with  the  mesial  and  distal  margins 
converging  one  way  or  the  other,  as  the  case  may  be.  The  marginal 
ridges  are  formed  in  a  manner  similar  to  those  of  the  first  molar,  with 
the  exception  of  the  distal  portion  of  the  buccal  ridge,  which  is  not 


15  £   6  rtJK 

Fig.  133. — Lower  Second  Molar,  Occlusal 
Surface. 


*  When  five  cusps  are  present,  the  anatomy  of  this  surface  does  not  differ  from  that  of  the 
first  molar. 


THE    LOWER    MOLARS 


185 


Distobuccal  Buccal   Mesiobuccal 
Cusp         Groove         Cusp 


Fig.  134. — Lower  Second  Molar, 
Buccal  Surface. 


broken  by  a  developmental  groove.  Each  marginal  ridge  is  divided 
near  its  center  by  one  of  the  grooves  of  development,  the  mesial  groove 
crossing  the  mesiomaiginal  ridge,  the  buccal  groove  crossing  the  bucco- 
marginal  ridge,  the  lingual  groove 
crossing  the  lingual  ridge,  and  the 
distal  groove  passing  over  the  distal 
ridge.  In  many  instances  numerous 
supplemental  grooves  are  present, 
which  in  turn  form  a  number  of 
smaller  ridges.  The  four  cusps  are 
the  mesiobuccal,  distobuccal,  mesio- 
lingual,  and  distolingual.  In  a  gen- 
eral way  they  are  similar  to  the  cusps 
of  the  first  lower  molar,  excepting 
that  they  are  somewhat  larger  and 
probably  less  pointed  and  less  angu- 
lar. Each  cusp  is  provided  with  a 
number  of  ridges,  which  descend  from 

the  summit  to  the  base,  two  of  these  contributing  to  the  formation  of 
the  marginal  ridges,  one  passing  to  the  buccal  or  lingual,  and  one,  the 
d  triangular  ridge,  descending  the  cen- 

tral incline  of  each  cusp.  The  names 
given  to  these  various  ridges  are  iden- 
tical with  those  of  the  first  molar. 

The  Buccal  Surface  of  the 
Crown  (Fig.  134). — The  principal 
difference  between  this  and  the  cor- 
responding surface  of  the  first  molar 
is  that  produced  by  the  absence  of  the 
fifth  cusp,  the  surface  being  divided 
into  two  parts  instead  of  three.  The 
single  division  is  caused  by  the  buccal 
groove,  which  reaches  the  surface  after 
crossing  the  buccomarginal  ridge 
from  the  occlusal  surface.  The  posi- 
tion of  this  groove  is  usually  a  little 
to  the  mesial  of  the  center  of  the 
surface.  Like  the  buccal  groove  of  the  first  molar,  it  may  disappear 
gradually  as  it  passes  toward  the  cervical  line,  or  it  may  end  in  a  well- 
marked  pit — the  buccal  pit. 


00  O.  EL,    ac  O. 

c  3  -  .E  3 


Fig.  135. — Lower  Second 
Molar,  Lingual  Surface. 


i86 


ANATOMY 


The  Lingual  Surface  of  the  Crown  (Fig.  135). — This  surface 
so  closely  resembles  the  corresponding  surface  of  the  first  molar  that 
it  is  somewhat  difficult  to  distinguish  one  from  the  other.     The  occlusal 


Mesiobuc- 
cal  Angle 


Lingual 
Groove 


Fig.  136. — Lower  Second  Molar,  Mesial  Surface. 

margin  may  be  a  trifle  less  irregular,  and  in  some  instances  more  ex- 
tensive, than  the  buccal  surface,  this  latter  feature  seldom  occurring  in 
the  first  molar. 

Mesial  Groove 


Buccal 
Groove 


Fig.  137. — Lower  Second  Molar,  Distal  Surface. 

The  Mesial  Surface  of  the  Crown  (Fig.  136). — This  surface  cor- 
responds to  the  mesial  surface  of  the  first  molar,  being  flattened  or 
slightly  convex  from  buccal  to  lingual,  with  an  inclination  to  a  slight 
depression  or  concavity  near  the  cervical  margin. 


THE    LOWER    MOLARS 


l87 


The  Distal  Surface  of  the  Crown  (Fig.  137). — On  account  of 
the  absence  of  the  fifth  cusp,  this  surface  is  less  complex  than  that  of 
the  first  molar.  It  is  convex  in  all  directions;  in  most  instances  smooth, 
in  others  broken  by  the  distal  groove,  which  reaches  it  after  crossing 
the  distomarginal  ridge  from  the  occlusal  surface. 

The  Roots  of  the  Tooth. — Like  the  first  molar,  these  are  two  in 
number,  a  mesial  and  a  distal.  They  are  much  less  constant  in  form, 
are  often  nearer  together,  and  in  some  instances  united.  When  the 
two  roots  exist — which  may  be  considered  the  normal  condition — they 
are  less  flattened  upon  their  mesial  and  distal  sides,  with  the  longitudinal 
depression  wanting  or  but  slightly  apparent.  These  roots,  therefore, 
are  more  rounded  in  general,  taper  more  gradually  from  neck  to  apex, 
and  end  in  a  rounded  apex,  this  often  being  provided  with  a  slight  distal 
curve. 

LOWER  THIRD  MOLAR. 


Calcification  Begins,  Eighth  to  Ninth  Year. 

Calcification  Completed,  Eighteenth  Year. 
Erupts,  Sixteenth  to  Twentieth  Year. 
Average  Length  of  Crown,  .26. 

Average  Length  of  Roots,  .36. 

Average  Length  over  All,  .62. 

This  tooth  is  probably  subject  to  a  greater  variety  in  form  than 
any  other.     There  are,  however,  two  varieties  which  are  most  frequently 

met  with.  In  one  the  crown  of  the 
tooth  is  similar  to  the  lower  second 
molar,  being  provided  with  four 
cusps,  which  are  separated  from  one 
another  by  four  developmental 
grooves  (Fig.  138).  The  other  is 
to    the    lower    first  molar, 


Disto- 

linguai  similar 

Groove 

Bucco-  having  five  cusps  and  five  develop- 

mar- 

ginai     mental  grooves. 

Ridge  ° 

While  these  two  forms  are  those 
most  commonly  met  with,  the  oc- 
clusal surface  may  be  so  broken  by 
numerous  supplemental  and  devel- 
opmental grooves  that  even  six  or 
eight  well-defined  cusps  may  be 
present.  Whatever  complications  may  exist  upon  the  occlusal  surface, 
a  central  fossa  is  usually  present,  from  which  radiate  the  various  develop- 


Disto- 
mar- 
ginal 
Ridge 


Mesio- 
mar- 
ginal 

Ridge 


mo 
Fig.  138. — Lower  Third  Molar, 
Occlusal  Surface. 


158  ANATOMY 

mental  grooves.  When  the  central  fossa  is  absent,  the  space  which  it 
should  occupy  is  usually  taken  up  by  a  rounded  cusp,  by  the  interference 
of  which  the  grooves  are  prevented  from  uniting,  and  their  course  is 
much  distorted.  Along  with  these  variations,  the  tooth  is  subject  to 
much  variety  in  size.  In  some  instances  the  crown  is  one-third  less 
in  circumference  than  that  of  either  the  first  or  second  lower  molars, 
while  in  others  it  is  a  trifle  greater.  The  increase  in  the  size  of  the  crown 
is  generally  accompanied  by  an  increase  in  the  number  of  cusps.  One 
feature  very  common  to  the  crown  is  its  inclination  to  the  circular  form, 


Disto-      Disto-     Mesio- 

lingual     buccal    buccal 

Cusp         Cusp       Cusp 


Buccal 
Groove 


Fig.  139. — Lower  Third  Molar, 
Buccal  Surface. 


Fig.  140. — Lower  Third  Molar, 
Lingual  Surface. 


almost  resulting  in  the  absence  of  the  angles  common  to  molars  in  general. 
The  marginal  ridges  are,  of  course,  subject  to  the  ever-varying  con- 
ditions to  be  found  upon  the  occlusal  surface;  in  general,  they  are  poorly 
defined,  and  are  frequently  crossed  by  numerous  small  supplemental 
grooves,  dividing  them  into  many  minute  tubercles.  The  latter  are 
smooth  and  strongly  convex,  with  their  general  outlines  much  influenced 
by  the  number  of  cusps.  ^ 

The  Roots  of  the  Tooth. — While  this  tooth  is  strongly  inclined 
to  be  two-rooted,  like  the  other  lower  molars,  this  condition  is  by  no 
means  the  common  one.  Like  the  crown,  the  roots  are  probably  more 
variable  than  those  of  any  other  tooth.     A  single  conic  root  may  be 


THE    LOWER    MOLARS  189 

present,  or  a  mesial  and  a  distal  root  may  exist;  again,  the  mesial  root 
may  bifurcate,  thus  resulting  in  three.  In  some  instances,  four,  or  even 
five,  branches  may  be  given  off  from  a  common  base.     When  more 


Fig.   141.— Types  of  Lower  Third  Molars. 

than  two  roots  are  present,  they  are  usually  much  twisted  or  crooked, 
and,  while  generally  inclined  to  the  distal,  are  liable  to  branch  in  various 
directions. 

A  better  idea  of  the  variations  in  this  tooth  may  be  had  from  the 
accompanying  illustration  (Fig.  141). 


CHAPTER  X. 

The  Pulp-cavities  of  the  Teeth. 

In  the  preceding  chapters  the  study  of  the  teeth  has  been  confined 
to  their  external  forms;  it  will  now  be  necessary  to  learn  something  of 
their  internal  anatomy,  and  for  this  purpose  various  dissections  of  each 
individual  tooth  must  be  made. 

Dissections. — First,  a  longitudinal  dissection  of  each  tooth  should 
be  made  by  sawing  or  filing  from  labial  to  lingual  in  the  anterior  teeth, 
or  from  buccal  to  lingual  in  the  posterior  teeth.  Second,  a  longitudinal 
dissection  by  sawing  from  mesial  to  distal.  Third,  numerous  transverse 
dissections  by  sawing  through  the  crown  or  root  at  various  points. 

These  dissections  will  expose  to  view  a  central  cavity  with  outlines 
closely  corresponding  to  those  of  the  tooth  itself.  This  is  called  the 
pulp-cavity,  and  in  the  vital  tooth  contains  the  formative  and  life-sustain- 
ing substance  of  the  dentin,  the  dental-pulp.  The  pulp-cavity  is  divided 
into  two  principal  parts,  that  portion  within  the  crown  of  the  tooth  being 
the  pulp-chamber,  while  that  traversing  the  root  is  the  pulp-canal.  At 
the  apex  of  the  root  the  canal  ends  in  a  small  foramen,  the  apical  fora- 
men* which  transmits  the  blood-vessels  and  nerves  to  the  pulp.  The 
pulp-chamber  occupies  the  center  of  the  crown  and  is  always  a  single 
cavity;  the  pulp-canals  are  prolongations  from  this  central  cavity,  and 
are  usually  one  for  each  root,  although  in  some  instances  two  or  more 
canals  are  present  in  a  single  root.  The  form  of  the  pulp-chamber 
varies  with  the  shape  of  the  crown,  the  outline  of  the  cutting-edge  in  the 
incisor  teeth  being  leproduced  in  that  part  of  the  chamber  nearest  to  the 
cutting-edge,  while  in  the  bicuspids  and  molars  the  occlusal  surface  is 
reproduced  on  the  wall  of  the  pulp-chamber,  immediately  beneath  it, 
the  lateral  walls  corresponding  to  the  various  sides  of  the  tooth.  In 
the  incisors  and  cuspids  the  pulp-chamber  passes  so  gradually  into  the 
pulp-canal  that  a  positive  line  of  demarcation  between  the  two  is  not 
observed.  In  the  bicuspids  and  molars  the  canals  may  be  readily 
distinguished  by  a  sudden  constriction  and  branching  out  of  the  cavity 
into  the  various  roots,  which  prolongations  gradually  decrease  in  size 

*  In  many  instances  there  is  more  than  one  foramen. 

190 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  191 

until  the  apical  foramen  is  reached.  The  size  of  the  pulp-cavity  is  much 
influenced  by  the  age  of  the  tooth,  its  functional  activity,  character  of 
the  occlusion,  etc.  The  tooth-pulp,  as  the  formative  organ  of  the  dentin, 
gradually  decreases  in  size  as  the  tooth  develops  (see  Development  of 
the  Teeth),  and  as  a  result  of  this  action  the  youngest  teeth  are  provided 
with  the  largest  pulp-cavities.  At  the  time  of  eruption  of  a  tooth,  the 
diameter  of  the  pulp-cavity  is  about  equal  to  one-half  the  diameter  of 
the  crown,  while  the  length  of  the  canal  must,  of  necessity,  accord  with 
the  extent  of  root-calcification.  As  the  growth  of  the  tooth  proceeds, 
the  diameter  of  both  the  chamber  and  canal  is  gradually  diminished; 
this  gradual  reduction  in  size  is  continued  during  the  life  of  the  tooth, 
and  if  permitted  to  proceed  until  old  age,  the  chamber  and  canal  may 
become  almost  or  entirely  obliterated.  It  must  be  remembered  that 
while  the  diameter  of  the  root-canal  is  diminished  with  the  growth  of 
the  tooth,  its  length  increases,  continuing  to  do  so  until  the  time  of  com- 
plete root-calcification.  During  the  period  of  root-development  the 
diameter  of  the  root-canal  is  greatest  at  the  free  or  apical  end  of  the  root, 
at -which  point  it  presents  a  funnel-shaped  opening  (Fig.  142).  As  the 
root  continues  to  calcify,  this  funnel-shaped  extremity  of  the  canal 
advances  in  the  direction  of  calcification,  and  finally,  as  the  formative 
process  nears  completion,  the  mouth  of  the  funnel  gradually  disappears, 
and  the  apical  foramen  is  established.  The  various  lobes  of  the  teeth 
are  penetrated  by  a  prolongation  of  the  pulp-cavity,  these  being  called 
the  horns  of  the  pulp-chamber.  The  depth  to  which  the  horn  penetrates 
the  lobe  varies  in  accordance  with  the  form  of  the  latter.  If  the  tooth 
is  one  provided  with  long,  penetrating  cusps,  the  horns  of  the  pulp- 
chamber  will  also  be  long,  but  if  the  cusps  be  poorly  formed,  the  horns 
of  the  chamber  will  be  short.  In  the  anterior  teeth,  when  the  lobal 
construction  is  outlined  by  well-marked  developmental  grooves,  the 
horns  of  the  pulp-chamber  will  be  three  in  number  and  directed  toward 
the  cutting-edge.  These  are  most  marked  in  young  teeth,  and  gradually 
disappear  as  age  advances.  The  functional  activity  of  the  teeth  also 
serves  to  materially  reduce  the  size  of  the  pulp-chamber.  Thus,  when 
opposing  teeth  occlude  squarely  and  firmly  against  each  other,  with  more 
or  less  rubbing  or  sliding  during  mastication,  the  external  surface  is 
prone  to  rapid  abrasion,  and,  as  a  direct  result  of  this,  external  change, 
the  pulp-chamber  undergoes  a  corresponding  alteration  by  a  growth 
of  secondary  dentin  about  its  walls. 


I92  ANATOMY 

THE  PULP-CAVITIES  OF  THE  UPPER  TEETH. 

Upper  Central  Incisor. — Figure  142  represents  a  number  of  la- 
biolingual  sections  presenting  the  relative  size  and  shape  of  the  pulp- 
cavity  in  the  upper  central  incisor  at  various  ages.  In  No.  1  the  con- 
dition existing  at  about  the  sixth  year,  or  at  a  time  immediately  prior 
to  the  eruption  of  the  tooth,  is  shown.  The  tooth-crown  is  fully  formed 
and  calcified;  the  cervical  line  may  be  observed,  as  well  as  a  small  portion 
of  the  root-wall.     The  pulp-chamber,  which  is  represented  by  the  dark 


Fig.  142. — The  Pulp-cavity  in  the  Upper  Central  Incisor,  from  the  Sixth  to 
the  Tenth  Year. 

portion  of  the  cut,  occupies  about  one-third  of  the  diameter  of  the  crown 
at  its  greatest  width.  The  pulp-chamber  at  this  age,  when  viewed 
in  this  direction,  forms  almost  a  perfect  cone,  the  base  of  which  is  directed 
upward  or  toward  the  future  extremity  of  the  root,  and  its  apex  down- 
ward in  the  direction  of  the  cutting-edge  of  the  tooth.  The  apex  of  the 
cone  may  end  somewhat  abruptly,  or  it  may  be  lengthened  into  a  slender, 
horn-like  projection,  extending  well  toward  the  cutting-edge.  No.  2 
represents  the  same  tooth  about  the  seventh  year,  or  at  a  time  shortly 
after  its  eruption.  The  pulp-chamber  has  become  slightly  reduced  in 
its  basal  diameter,  while  but  little  change  has  taken  place  in  the  apex. 
That  portion  of  the  pulp-cavity  above  the  cervical  line  represents  a  part 
of  the  future  pulp-canal.  At  this  age  the  canal  is  a  direct  continuation 
of  the  conic  pulp-chamber,  ending  above  in  a  broad,  funnel-shaped 
extremity.  No.  3  shows  the  condition  of  the  cavity  about  the  eighth  year. 
The  diameter  of  the  pulp-chamber  is  considerably  diminished,  the  apex 
has  slightly  receded,  and  the  horn-like  projection  has  partly  disappeared. 
The  increase  in  the  length  of  the  canal  is  about  3/16  of  an  inch  over  its 
length  at  seven  years.     The  two  parallel  sides  of  the  canal  have  lengthened 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  1 93 

proportionately,  and  the  funnel-shaped  extremity  is  reduced  in  diameter 
owing  to  the  gradual  narrowing  of  the  roots-wall.  No.  4  gives  the 
relative  size  of  the  pulp-chamber  and  canal  at  the  ninth  year,  or  at  a 
time  when  root-calcification  is  nearing  completion.  The  decrease  in 
the  capacity  of  the  chamber  is  readily  apparent;  the  horn-like  projection 
has  disappeared  and  the  parallel  sides  of  the  canal  are  partly  extended 
into  the  chamber,  thus  reducing  the  length  of  the  cone.  In  the  canal  a 
greater  reduction  has  taken  place  in  its  diameter,  while  its  length  has 
increased  about  1/4  of  an  inch  over  that  at  eight  years,  and  the  diameter 
of  the  funnel-shaped  opening  is  but  little  greater  than  that  of  the  body 


Fig.  143. — Pulp-cavity  in  the  Upper  Central  Incisor. 

of  the  canal.  No.  5,  which  represents  a  section  of  the  tooth  about  the  tenth 
year,  shows  calcification  in  the  root  completed,  and  the  apical  foramen 
established.  A  glance  at  the  illustration  will  show  the  gradual  decrease 
in  the  capacity  of  the  pulp-cavity  and  the  completion  of  its  growth  in  an 
apical  direction.  At  this  stage  of  development  the  fan-shaped  extremity 
of  the  canal  gradually  disappears,  and  for  the  first  time  in  the  life  of  the 
tooth  the  canal  partakes  of  the  external  root  form  throughout  its  entire 
extent. 

Figure  143  A,  represents  the  size  and  form  of  the  average  pulp- 
cavity  in  the  adult  upper  central  incisor.  In  its  entirety  it  represents  a 
double  cone,  with  a  common  base  near  the  cervical  line,  the  pulp-cavity 
forming  one  cone  and  the  pulp-canal  the  other.  At  this  common  base 
the  cavity  assumes  its  largest  diameter,  which  measurement  is  approxi- 
mately equal  to  one-fourth  the  labiolingual  diameter  of  the  tooth.  The 
extent  and  "form  of  the  lower  cone,  or  that  represented  by  the  pulp- 
chamber,  varies  in  the  adult  tooth  with  the  tooth  type.  Thus,  in  the 
nervous  type  the  cone  is  long  and  narrow,  with  the  apex  ending  in  a  hair- 


194  ANATOMY 

like  projection.  In  the  tooth  of  the  lymphatic  temperament  the  cone 
is  prone  to  be  wide,  with  its  apex  ending  abruptly.  In  the  sanguine  and 
bilious  types  the  form  and  extent  of  the  cone  do  not  partake  of  either 
of  the  foregoing  extremes,  but,  in  keeping  with  the  outline  of  the  crowns, 
are  intermediate  between  them.  Figure  143  B,  represents  the  average 
condition  of  the  pulp-cavity  in  the  central  incisor  in  advanced  age,  and 
shows  a  general  reduction  in  the  size  of  both  chamber  and  canal.  A 
further  study  of  the  pulp-chamber  and  canal  may  be  made  by  a  mesio- 
distal  section  made  through  the  long  axis  of  the  tooth  (Fig.  143  C). 
The  outline  of  the  cavity,  viewed  in  this  way,  closely  follows  the  outline 
of  the  crown  and  root  of  the  tooth.  There  is  no  distinct  division  between 
the  chamber  and  the  canal,  the  former  gradually  blending  into  the  latter. 
The  outline  of  the  entire  cavity  is  that  of  a  single  cone,  with  its  base 
directed  toward  the  cutting-edge  and  its  apex  in  the  direction  of  the 
apical  extremity  of  the  root.  The  lower  margin  of  the  pulp-chamber, 
or  that  nearest  the  cutting-edge  of  the  crown,  is  broad  from  mesial  to 
distal  and  thin  from  labial  to  lingual.  This  margin  in  the  average  adult 
tooth  is  about  on  a  line  with  the  center  of  the  labial  surface  of  the  crown, 
and  the  lateral  walls  of  the  cavity  as  they  pass  upward  converge  slightly, 
and  finally  blend  into  the  walls  of  the  canal,  at  a  point  somewhat  beyond 


Fig.  144. — Transverse  Sections,  Root  of  Upper  Central  Incisors,  Slightly 

Enlarged. 

the  cervical  line.  During  the  early  life  of  the  tooth  the  margin  of  the 
chamber  nearest  the  cutting-edge  presents  three  well-defined  horns, 
corresponding  to  the  three  rudimentary  lobes  found  upon  the  cutting- 
edge  at  this  period.  These  horns  rapidly  disappear,  and  are  seldom 
found  after  the  fifteenth  year.  In  certain  tooth  types,  however,  the 
mesial  and  distal  horns  may  continue  present  until  adult  age,  and  even 
into  mipMle  life,  but  when  this  occurs  it  is  not  the  result  of  the  temporary 
tooth  form,  but  is  occasioned  by  the  permanent  angular  outline  of  the 
crown. 

Figure  144  represents  a  number  of  transverse  sections  of  an  upper 
central  incisor,  showing  the  outline  and  relative  size  of  the  pulp-cavity 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  195 

in  passing  from  the  base  of  the  crown  toward  the  apex  of  the  root.  No.  i 
shows  the  outline  of  the  cavity  at  the  cervical  line;  No.  2  represents  the 
condition  1/8  of  an  inch  nearer  the  apex  of  the  root;  No.  3  is  from  the 
center  of  the  root  length,  while  No.  4  is  from  the  region  of  the  apex. 

Upper  Lateral  Incisor. — The  pulp-cavity  in  the  upper  lateral 
incisor  is  so  nearly  identical  with  that  of  the  central  that  it  will  only  be 
necessary  to  call  attention  to  one  or  two  points  which  are  at  variance. 
Figure  145  shows  the  five  stages  as  represented  by  the  growth  of  the 
tooth.  In  general  it  will  be  observed  that  the  cavity  is  much  smaller 
than  that  of  the  central  incisor,  but  this  difference  is  to  be  accounted  for 


Fig.  145. — Pulp-cavity  in  the  Upper  Lateral  Incisor,  from  the  Sixth  to  the 

Tenth  Year. 

in  the  smaller  proportions  of  the  tooth.  No.  1  shows  the  condition  of 
the  crown  and  pulp-cavity  about  the  sixth  year,  the  pulp-cavity  occupying 
a  large  portion  of  the  partly  clacified  tooth-crown.  No.  2  represents 
the  conditions  present  at  the  seventh  year,  or  about  the  time  of  the 
eruption  of  the  tooth.  The  pulp-chamber  at  this  age  resembles  a  perfect 
cone,  the  base  of  which  reaches  to  the  root-walls,  and  faintly  outlines 
the  beginning  of  the  future  pulp-canal.  In  No.  3,  at  eight  years,  the 
length  of  the  root  has  increased  about  3/16  of  an  inch,  and  the  parallel 
sides  of  the  walls  of  the  pulp-canal  have  made  their  appearance.  In  No. 
4,  at  nine  years,  by  the  growth  of  the  root  the  canal  has  considerably 
increased  in  length  and  at  the  same  time  much  decreased  in  diameter, 
while  in  No.  5,  at  ten  years,  the  root  is  completely  formed,  the  apical 
foramen  established,  and  the  maximum  size  of  the  entire  pulp-cavity 
in  the  fully  formed  tooth  shown. 

Figure  146,  A,  shows  the  average  condition  of  the  pulp-cavity  in 
the  upper  lateral  incisor  at  adult  age,  while  figure  146,  B,  represents 
the  same  tooth  in  old  age.  In  a  mesiodistal  section — figure  146,  C — a 
very  close  resemblance  to  the  pulp-cavity  in  the  central  incisor  will  be 


196  ANATOMY 

noticed.  While  the  pulp-cavity  is  smaller  than  that  of  the  central 
incisor,  it  is  usually  a  trifle  larger  in  proportion  to  the  size  of  the  tooth. 
Owing  to  the  marked  constriction  at  the  neck  of  this  tooth,  there  is 
occasionally  found  a  slight  line  of  distinction  between  the  pulp-chamber 
and  canal,  but  in  the  majority  of  instances  this  is  not  to  be  observed. 
The  horns  of  the  pulp-chamber  are  in  every  respect  similar  to  those 
of  the  central  incisor,  excepting  when  they  exist  permanently,  in  which 
case  the  mesial  horn  is  usually  the  longest.  By  the  transverse  sections 
shown  in  figure  146,  the  gradual  decrease  in  size  and  change  in  form  in 
the  root-canal  are  presented,  the  sections  being  similar  to  those  made 
in  the  root  of  the  central  incisor. 


Fig.  146. 

Upper  Cuspid. — The  pulp-cavity  of  this  tooth  is  in  general  similar 
to  that  of  the  incisors,  excepting  that  the  coronal  extremity  of  the  chamber 
is  conic  and  inclined  to  a  horn-like  projection  which  penetrates  the  single 
cusp  of  the  tooth-crown  in  the  direction  of  its  summit.  Figure  147 
represents  a  number  of  labiopalatal  sections.  No.  1  shows  the  condi- 
tion of  the  pulp-cavity  about  the  seventh  year,  or  fully  five  years  before 
the  eruption  of  the  tooth.  The  pulp-chamber  partakes  of  the  cone  shape 
previously  referred  to,  but  the  margins,  instead  of  being  straight  lines, 
are  somewhat  bowed  or  concave,  thus  conforming  more  closely  to  the 
outline  of  the  crown.     The  central  horn  of  the  chamber  is  proportionately 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  197 

longer  than  that  of  the  incisors,  in  correspondence  with  the  cusp  of  the 
tooth.  At  this  age  the  formative  process  has  barely  extended  to  the 
root-walls;  therefore,  the  width  of  the  cavity  is  about  equal  to  its  length. 
In  No.  2,  at  eight  years,  an  increase  in  the  capacity  of  the  chamber  over 
that  of  the  incisors  is  shown,  this  being  the  result  of  the  greater  bulk  in 
the.  tooth-crown.  The  cone-like  outline  of  the  chamber  is  somewhat 
broken  by  an  effort  of  its  margins  to  follow  the  outline  of  the  crown. 
In  No.  3,  at  nine  years,  the  principal  change  has  taken  place  in  the 
canal,  which  has  lengthened  fully  3/16  of  an  inch,  and  the  funnel-shaped 
extremity,  instead  of  joining  with  the  pulp-chamber  direct,  is  continued 
below  by  two  parallel  walls  to  the  true  beginning  of  this  cavity.     At  ten 


■  V?      .^fl      ■  i  UF  ■     l»     ■■■':- 

B \m   YWLM  IJr 

mk    A         B    jtm         WW  A 

W  ■  \^i 

HfJ 

B\  JB 

Hk  JR 

Fig.  147. — Pulp-cavity  in  the  Upper  Cuspid,  from  the  Seventh  to  the  Twelfth 

Year. 

years,  No.  4,  a  more  marked  transformation  has  taken  place  in  both 
portions  of  the  cavity.  The  diameter  of  the  chamber  at  the  cervical  line 
has  diminished,  as  has  also  the  length  of  the  cone.  The  increase  in  the 
length  of  the  root,  which  has'been  proportionately  greater  than  that  of 
the  preceding  year,  has  extended  the  length  of  the  canal  about  3/8  of 
an  inch.  The  walls  of  the  canal  are  no  longer  parallel  with  each  other, 
but  are  inclined  to  follow  the  root-outlines.  The  funnel-shaped  opening 
is  much  reduced  both  in  length  and  breadth.  No.  5  represents  the 
condition  at  the  time  of  the  eruption  of  the  tooth,  or  about  the  twelfth 
year.  The  general  outline  of  the  pulp-cavity  is  that  of  a  double  cone, 
with  a  common  base  at  a  point  nearly  corresponding  to  the  cervical  line. 
The  diameter  in  both  the  chamber  and  canal  has  considerably  decreased, 
while  the  central  horn  in  the  former  has  further  receded.  The  calcifica- 
tion of  the  root  externally  is  about  complete  and  the  foramen  formed. 
In  this  particular  the  cuspid  tooth  differs  from  the  incisors,  and  in  fact 
from  all  other  teeth,  in  having  its  root-calcification  about  completed  and 


198 


ANATOMY 


the  apical  foramen  established  at  or  soon  after  the  time  of  its  eruption. 
Figure  148,  A,  gives  an  idea  of  the  capacity  of  the  pulp-cavity  in  the  upper 
cuspid  at  maturity,  while  figure  148,  B,  shows  the  condition  in  advanced 
age.  Figure  148,  C,  is  a  mediodistal  section  of  a  matured  upper  cuspid. 
The  coronal  extremity  of  the  pulp-chamber  is  square,  and  but  little 
inclined  to  follow  the  outline  of  the  mesial  and  distal  cutting-edges. 
The  chamber  passes  into  the  canal  without  a  mark  of  separation,  and 
the  latter  gradually  diminishes  in  diameter  as  the  apex  of  the  root  is 


D 


G 


Fig.  it 


approached.  At  its  point  of  beginning  the  canal  is  sometimes  inclined 
to  flatness  from  mesial  to  distal,  but  in  passing  toward  the  apex  this 
tendency  disappears,  and  it  becomes  more  circular  in  outline.  In  figure 
148,  D,  a  transverse  section  through  the  tooth  at  the  cervical  line  is 
shown,  giving  an  idea  of  the  proportionate  size  and  form  of  the  canal  in 
the  adult  tooth,  while  E,  F,  and  G  represent  transverse  sections  through 
the  root  of  the  same  tooth  at  various  points  between  the  cervical  line 
and  the  apex  of  the  root. 

Upper  First  Bicuspid. — The  study  of  the  pulp-cavity  in  this  tooth 
differs  in  many  particulars  from  that  of  the  incisors  and  cuspids.  First, 
the  line   of   distinction  between   pulp-chamber  and   the  root-canal  or 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  1 99 

canals  is,  in  most  instances,  definitely  marked  by  the  bifurcation  of 
the  roots  and  a  corresponding  branching  of  the  pulp-cavity  into  two 
fine  canals,  one  of  which  occupies  the  center  of  each  root.  This  division 
of  the  cavity  brings,  the  center  of  the  pulp-chamber  almost  on  a  level 
with  the  cervical  line.  In  figure  149,  No.  1  shows  the  partly  calcified 
crown  of  the  upper  first  bicuspid  at  the  seventh  year.  A  portion  of  the 
pulp-chamber  alone  may  be  studied  at  this  period,  and  this  is  found  to 
be  somewhat  irregular  in  outline,  with  a  broadened,  funnel-shaped  open- 
ing above,  and  two  small,  cone-like  projections  below,  pointing  into 
either  cusp  of  the  crown.  These  latter  projections  are  the  horns  of  the 
pulp-chamber,  and  are  named  in  accordance  with  the  cusp  which  they 


Fie  149. — Pulp-cavities  of  the  Upper  First  Bicuspid,  from  the  Seventh  to  the 

Twelfth  Year. 

occupy.  In  very  young  teeth  it  is  not  unusual  to  find  these  horns  pene- 
trating the  dentin  almost  to  the  enamel-wall.  No.  2,  at  eight  years, 
shows  the  crown  fully  calcined  and  the  outline  of  the  base  of  the  roots 
established.  The  horns  of  the  pulp-chamber  have  slightly  receded,  and 
the  branching  of  the  canals  is  made  manifest  by  the  central  deposit  of 
dentin.  In  No.  3,  at  nine  years,  the  capacity  of  the  pulp-chamber  is 
much  decreased,  and  appears  to  have  receded  bodily  rootward.  The 
roots  are  calcified  to  about  one-third  their  full  length,  and  the  canals 
which  traverse  them  are  each  provided  with  the  funnel-shaped  opening 
at  their  free  calcifying  extremities.  In  No.  4,  at  ten  years,  the  decrease 
in  the  size  of  the  pulp-chamber  is  not  only  caused  by  the  deposit  of 
dentin  upon  the  occlusal  and  lateral  walls,  but  from  the  direction  of  the 
roots  as  well.  The  diameter  of  the  root  canals  is  much  less  than  at  nine 
years,  but  the  walls  are  as  yet  parallel.  No.  5  shows  the  roots  fully 
formed  and  the  apical  foramina  established,  which  condition  occurs 
about  the  twelfth  year.     The  horns  of  the  pulp-chamber  have  receded 


200 


ANATOMY 


somewhat,  and  the  center  of  this  cavity  is  now  almost  on  a  level  with 
the  cervical  line.  The  canals  have  assumed  the  form  of  the  roots  them- 
selves, and  their  diameter  is  much  diminished.  The  illustration  shows 
the  proportionate  maximum  size  of  the  chamber  and  canals  in  this  tooth 
after  completion  of  surface  calcification.  It  will  be  observed  that  the 
foramina  are  proportionately  smaller  than  those  of  the  incisors  and 
cuspids  at  a  corresponding  period,  this  condition  resulting  from  the 
lesser  diameter  of  the  roots. 

Figure  150,  A,  illustrates  the  approximate  size  and  form  of  the  pulp- 


i) 


E  F  G 

Fig.  150. — Pulp-cavities  of  the  Upper  First  Bicuspid,  Enlarged  about  One-third. 


chamber  and  canals  at  adult  age,  and  attention  is  called  to  the  appear- 
ance of  the  horns  of  the  pulp-chamber.  It  will  be  observed  that  the 
horn  which  penetrates  the  buccal  cusp  is  larger  and  more  pointed  than 
that  directed  toward  the  lingual  cusp;  this  condition  is  fully  explained 
by  the  buccal  cusp  being  proportionately  larger  and  longer  than  the 
lingual.  In  the  same  figure,  B  represents  the  pulp-cavity  in  the  first 
upper  bicuspid  at  advanced  age.  The  foregoing  description  applies 
only  to  the  two-rooted  bicuspids,  but  as  many  of  these  teeth  have  but 
one  root,  an  additional  description  will  be  necessary.  When  a  single 
root  is  present,  many  varieties  in  the  outline  of  the  pulp-cavity  will 
be  presented;  this  variation,   however,  seldom   affects  the  capacity  or 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  201 

form  of  the  pulp-chamber.  Two  distinct  canals  may  exist  in  the  single 
root  (Fig.  150,  C),  branching  off  from  the  chamber,  one  from  the  buccal  and 
one  from  the  lingual  portion.  These  canals  gradually  taper  in  the  direc- 
tion of  the  apex  of  the  root,  and  may  end  in  a  single  foramen,  or  in  distinct 
foramina.  Occasionally  the  canals  will  unite  before  reaching  the  root- 
apex  and  continue  as  a  single  canal  ending  in  a  single  foramen,  or  they  may 
communicate  at  one  point  and  again  diverge  and  finally  end  in  separate 
foramina.  In  some  instances  the  pulp-canal  appears  to  be  a  direct 
continuation  of  the  pulp-chamber,  extending  throughout  the  length  of 
the  root  in  the  form  of  a  flattened  canal,  with  its  greatest  diameter  from 
buccal  to  lingual  (Fig.  150,  D).  When  two  separate  canals  exist  in  the 
single  root,  the  outward  appearance  of  the  root  indicates  a  near  approach 
to  two  roots;  when  the  single  flattened  canal  is  present,  the  root  is  also 
flattened  and  shows  no  sign  of  bifurcation.  Reference  has  been  made 
to  the  horns  of  the  pulp-chamber,  and  in  this  connection  it  will  be  well 
to  speak  of  the  extent  to  which  they  may  exist.  In  that  type  of  tooth 
provided  with  long  penetrating  cusps  the  horns  will  dip  well  down  into 
the  cusp  occasionally  to  the  full  depth  of  the  dentin,  and  in  rare  instances 
may  penetrate  the  enamel.  In  those  teeth  lacking  in  cusp-formation 
the  length  of  the  horns  will  be  correspondingly  reduced,  and  may  be 
entirely  wanting.  In  the  two-rooted  upper  first  bicuspid  the  floor  of 
the  pulp-chamber,  or  that  part  of  the  cavity  directed  rootward,  is  promi- 
nent and  rounded  in  the  center,  from  which  point  it  gradually  slopes 
toward  the  entrances  to  the  canals,  one  of  which  arises  from  the  extreme 
buccal  margin,  and  the  other  from  the  extreme  lingual  margin.  Figure 
150,  E,  represents  a  transverse  section  of  the  two  roots  immediately 
below  the  point  of  bifurcation.  F  represents  a  section  of  the  two  roots 
midway  between  the  cervical  line  and  the  apical  extremity,  while  G  is 
a  transverse  section  of  D  at  the  cervical  line. 

Upper  Second  Bicuspid. — The  pulp-cavity  of  this  tooth  is  in 
many  respects  similar  to  that  of  the  first  bicuspid,  the  principal  variations 
being  in  the  horns  of  the  chamber,  which  are  proportionately  smaller 
in  correspondence  with  the  diminution  in  cusp-formation.  There  is 
usually  no  positive  line  of  demarcation  between  the  chamber  and  canal, 
the  latter  being  quite  large,  and  broad  from  buccal  to  lingual.  The 
extent  of  the  pulp-chamber  is  sometimes  well  defined  by  the  presence  of 
two  root-canals,  similar  to  those  described  in  connection  with  the  first 
bicuspid.  In  rare  instances  the  tooth  may  possess  two  roots,  each  of 
which  would  be  traversed  by  a  canal.  Figure  151  represents  the  various 
stages  of  the  development  of  the  pulp-cavity,  as  shown  by  a  longitudinal 


202 


\\  \  lu\n 


section  from  buccal  to  lingual.  No.  i  shows  the  condition  at  seven 
years,  or  at  a  time  when  a  portion  of  the  crown  only  is  calcified,  in  con- 
sequence of  which  the  pulp-chamber  alone  can  be  studied  at  this  period. 


Fig.  151. — Pulp-cavities  in  the  Upper  Second  Bicuspid,  from  the  Seventh  to 
the  Twelfth  Year. 

The  buccal  and  lingual  horns  of  the  chamber  may  be  observed  penetrat- 
ing the  dentin  in  the  direction  of  their  respective  cusps.  No.  2  shows 
the  advance  made  in  the  formative  process  by  the  eighth  year,  or  at  a 


Fig. 


152. 


A  B 

-Section  of  Upper  Second  Bicuspid,  Slightly  Enlarged. 


time  immediately  prior  to  the  eruption  of  the  tooth;  the  outline  of  the 
chamber  is  completed  and  the  walls  of  the  future  pulp-canal  faintly 
outlined.     At  this  period  there  has  been  but  little  change  in  the  horns 


THE   PULP-CAVITIES    OF    THE    UPPER   TEETH  203 

of  the  pulp-cavity.  No.  3  shows  the  condition  of  the  tooth  at  the  ninth 
year,  or  at  the  beginning  of  its  eruptive  period.  The  diameter  of  the 
chamber  has  somewhat  decreased,  the  horns  have  slightly  receded,  the 
funnel-shaped  extremity  of  the  cavity  has  advanced  beyond  the  cervical 
line,  and  is  now  confined  to  the  canal  alone.  No.  4  represents  the 
condition  of  the  pulp-cavity  about  the  tenth  year.  A  gradual  decrease 
in  the  diameter  of  both  the  chamber  and  canal  is  observed,  and  the  horns 
of  the  pulp-cavity  are  growing  less  prominent.  The  length  of  the  root 
having  increased  nearly  one-quarter  of  an  inch,  we  find  a  corresponding 
addition  to  the  length  of  the  canal.  No.  5  shows  the  maximum  size  of  the 
pulp-cavity  in  the  upper  second  bicuspid,  which  condition  accompanies 
the  completion  of  the  external  calcification  at  the  twelfth  year.  As 
previously  stated,  the  cavity,  in  its  entirety,  presents  no  line  of  separation 
between  the  chamber  and  canal,  but  gradually  tapers  from  its  broadened 
base  in  the  crown  to  its  ending  at  the  apex  of  the  root.  In  this  tooth, 
as  well  as  in  all  those  previously  described,  the  apical  foramen  at  the 
time  of  completion  of  root-calcification  is  comparatively  large,  and 
readily  penetrated  during  operations  upon  it.  Figure  152  illustrates 
a  number  of  sections  of  an  upper  second  bicuspid  at  maturity.  The 
same  figure  also  represents  two  transverse  sections,  A  being  at  the  cervical 
line,  B  midway  between  the  cervical  line  and  apex  of  the  root. 

PULP-CAVITIES  OF  THE  UPPER  MOLARS. 

The  inner  anatomy  of  the  molar  teeth  being  much  more  complicated 
than  any  of  those  previously  described,  it  will  be  found  necessary  to  make 
a  number  of  dissections  in  various  directions  in  order  to  obtain  a  com- 
prehensive idea  of  the  location  and  form  of  the  different  parts  of  the 
pulp-cavity.  The  line  of  demarcation  between  the  pulp-chamber  and 
canals  is  always  definite,  the  former  occupying  a  central  position  in  the 
crown  and  seldom  extending  beyond  the  cervical  line,  while  the  latter 
are  given  off  from  the  floor  of  the  chamber  and  penetrate  the  various 
roots,  their  entrances  being  marked  by  small  funnel-shaped  openings  in 
the  floor  of  the  chamber.  In  the  matured  tooth  the  form  of  the  chamber 
usually  corresponds  to  that  of  the  crown  of  the  tooth.  The  lateral  walls 
of  the  chamber  are  four  in  number,  and  are  named  according  to  their 
location — mesial,  distal,  buccal,  and  lingual.  The  average  thickness 
of  these  walls  at  maturity  is  about  equal  to  the  diameter  of  the  pulp- 
chamber.  In  that  type  of  tooth  common  to  the  lymphatic  temperament 
where  there  is  but  little  constriction  at  the  neck,  resulting  in  the  various 


204 


ANATOMY 


sides  of  the  tooth-crown  being  nearly  parallel  with  each  other,  the  pulp- 
chamber  is  nearly  quadrilateral  in  form;  but  in  those  teeth  marked  by  a 
decided  constriction  at  the  neck,  most  marked  in  the  nervous  tempera- 
ment, the  extent  of  surface  covered  by  the  floor  of  the  chamber  is  much 
less  than  that  occupied  by  the  occlusal  portion.  In  the  former  class, 
the  entrances  to  the  various  canals  are  much  farther  apart  than  in  the 
latter.  The  occluding  wall  is  usually  much  thicker  than  the  lateral 
walls,  and  is  penetrated  by  the  horns  of  the  pulp-chamber,  one  of  which 
extends  into  each  cusp.  As  in  the  bicuspids,  the  extent  to  which  the 
horns  penetrate  the  cusps  is  controlled  by  the  prominence  of  the  latter. 

The  floor  of  the  pulp-chamber  is  irregu- 
c  larly  rounded,  being  high  in  the  corner 
and  gradually  falling  away  in  the  direc- 
tion of  the  canals.  The  entrances  to  the 
B  root-canals,  three  in  number,  are  placed 
in  the  form  of  an  irregular  triangle,  called 
the  molar  triangle.  The  mesial  side  of 
the  triangle  is  usually  the  longest,  the  distal 
next  in  length,  and  the  buccal  the  shortest. 
In  young  teeth  the  entrances  to  the  canals 
are  usually  in  the  from  of  funnel-shaped 
openings,  are  comparatively  easy  of  access, 
bat  after  maturity  may  disappear  and  be  but  little  larger  than  the  canals 
themselves.  To  properly  study  the  position  occupied  by  the  entrance 
to  the  canals  on  the  floor  of  the  pulp-chamber,  a  transverse  section  of 
the  tooth  should  be  made  at  a  point  somewhat  above  the  cervical  line, 
at  the  same  time  preserving  both  the  crown  and  the  roots  of  the  tooth 
for  comparison.  The  entrance  to  the  lingual  canal,  which  is  usually 
the  largest  and  most  readily  accessible,  may  be  located  by  a  line  drawn 
through  the  center  of  the  occlusal  surface  of  the  crown  (Fig.  153)  from 
buccal  to  lingual,  A,  and  by  another  line  drawn  from  mesial  to  distal 
almost  parallel  with  the  linguomarginal  ridge,  passing  through  the 
summits  of  the  mesiolingual  and  distolingual  cusps,  B;  the  point  at  which 
these  two  lines  intersect  will  mark  the  approximate  location  of  the  lingual 
canal.  The  entrance  to  the  mesiobuccal  canal  may  be  located  by  a  line 
drawn  from  the  inner  side  of  the  mesiobuccal  angle  to  a  corresponding 
position  near  the  distobuccal  angle,  C.  This  should  be  intersected 
by  a  line  drawn  from  the  summit  of  the  mesiobuccal  cusp  to  the  summit 
of  the  mesiolingual  cusp,  D,  the  point  at  which  these  two  lines  cross 
marking  the  entrance   to  the  mesiobuccal   canal.     The  location  of  the 


Fig. 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  205 

entrance  to  the  distobuccal  canal  is  found  by  the  line,  C,  which  is  inter- 
sected by  another  line,  E,  drawn  from  the  summit  of  the  distobuccal 
cusp  to  a  corresponding  point  on  the  distolingual  cusp.  The  nearer 
the  tooth-crown  approaches  to  the  quadrilateral,  the  nearer  will  the 
molar  triangle  approach  the  equilateral. 

Upper  First  Molar. — In  the  dissection  of  this  tooth,  the  pulp-cham- 
ber and  two  of  the  root-canals  only  can  be  shown,  but  these  will  be 
sufficient  to  pursue  the  study  with  intelligence.  Figure  154  shows  a 
number  of  longitudinal  sections,  made  in  such  a  manner  as  to  expose 
the  Ungual  canal,  usually  the  largest,  and  the  mesiobuccal  canal.  No.  1 
illustrates  the  approximate  size  and  form  of  the  pulp-chamber  at  the 


Fig.  154. — Pulp-cavity  of  Upper  First  Molar,  from  the  Fifth  to  the  Ninth 
Year.     Lingual  and  Mesiobuccal  Canals. 

fifth  year.  At  this  period  the  chamber  occupies  a  large  proportion  of 
the  center  of  the  tooth-crown.  Two  of  the  four  horns  are  seen,  one  of 
which  penetrates  the  mesiobuccal  cusp,  and  one  the  mesiolingual  cusp. 
In  many  instances  the  horns  of  the  molar  teeth  are  quite  slender,  penetrat- 
ing the  dentin  to  a  greater  depth  than  shown  in  the  illustration,  in  the 
form  of  minute  hair-like  projections,  which  in  some  instances  reach 
almost  to  the  enamel  walls. 

No.  2  illustrates  the  condition  of  the  pulp-cavity  at  the  sixth  year, 
or  at  the  time  of  eruption.  The  outline  of  the  pulp-chamber  is  completed, 
and  the  floor  has  begun  to  make  its  appearance  by  a  central  deposit 
of  dentin.  It  will  be  observed  that  the  lateral  walls  of  the  chamber  are 
somewhat  less  in  thickness  than  the  occluding  wall,  a  condition  which 
will  become  more  pronounced  as  the  tooth  develops.  With  the  beginning 
of  the  formative  process  in  the  floor  of  the  chamber  we  find  the  trifurca- 
tion  of  the  roots  established,  and  the  beginning  of  the  canals  outlined. 
The  canals  at  this  period  are  quite  similar  to  those  of  the  bicuspid,  being 
provided  with  a  funnel-shaped  extremity,  which  extends  from  the  free 


206  ANATOMY 

calcifying  margins  of  the  roots  to  the  floor  of  the  chamber.  No.  3  shows 
the  change  which  has  taken  place  at  the  seventh  year.  While  the  pulp- 
chamber  is  somewhat  reduced  in  size,  but  little  change  is  noticeable 
in  its  outline.  By  this  time  the  floor  of  the  chamber  has  become  an 
important  factor  in  the  tooth  development.  By  the  constant  lateral 
extension  of  this  central  deposit  of  dentin  the  floor  of  the  chamber  is 
gradually  spread  out,  this  alteration  being  at  the  expense  of  the  entrances 
to  the  root-canals,  which  become  reduced  in  diameter  as  the  floor  is 
extended.  The  horns  of  the  chamber  are  slightly  less  prominent,  but  this 
part  of  the  cavity  has  the  appearance  of  having  receded  bodily  root- 
ward.  The  roots  have  advanced  somewhat  beyond  the  point  of  trifurca- 
tion,  and  a  definite  outline  has  been  given  to  the  canals.  At  this  period 
the  diameter  of  the  root-wall  is  about  equal  to  the  diameter  of  the  pulp- 
canal.  Along  with  the  gradual  decrease  in  the  diameter  of  the  roots, 
there  is  observed  a  corresponding  decrease  in  the  width  of  the  funnel- 
shaped  extremities  of  the  canals. 

At  the  eighth  year,  No.  4,  a  gradual  reduction  in  the  capacity  of  both 
the  chamber  and  canal  is  noted.  Accompanying  the  above  condition 
there  is  found  a  corresponding  increase  in  the  thickness  of  the  surround- 
ing walls.  The  horns  of  the  pulp-chamber  are  much  reduced  in  size, 
and  the  form  of  the  chamber  more  closely  resembles  that  of  the  general 
contour  of  the  tooth-crown.  The  increase  in  the  length  of  the  roots 
is  proportionately  greater  than  that  of  previous  years,  in  consequence 
of  which  the  length  of  the  canals  is  increased  to  a  greater  degree.  In 
No.  5  the  maximum  size  of  the  chamber  and  canals  is  apparent,  which 
condition  takes  place  about  the  ninth  year,  or  at  a  time  when  calcification 
of  the  tooth  is  completed  externally.  In  some  instances,  owing  to  the 
additional  length  of  the  lingual  root,  the  apical  foramen  may  not  be 
established  before  the  tenth  year.  At  this  latter  period  it  is  safe  to  assume 
that  all  three  canals  have  completed  their  longitudinal  extent,  and  the 
foramina,  although  proportionately  large,  have  been  established,  so 
that  a  more  definite  description  of  each  canal  may  be  given.  The  lingual 
canal  (Fig.  155,  A)  is  usually  the  largest,  and  branches  off  from  the  floor 
of  the  chamber,  near  the  mesiodistal  center  of  tl  j  extreme  lingual  margin, 
the  entrance  in  the  average  tooth  being  well  defined  by  a  circular,  funnel- 
shaped  opening.  The  direction  of  this  canal  is  usually  upward  and 
slightly  inward,  until  the  apical  extremity  is  approached,  at  which  point 
it  is  inclined  to  the  buccal.  The  circular  form  presented  at  the  beginning 
of  the  canal  is  generally  continued  throughout  its  entire  length,  in  this 
repect  differing  from  the  two  buccal  canals.     The  average  length  of 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  207 

the  lingual  canal  is  about  1/2  of  an  inch.  The  mesiobuccal  canal  (Fig. 
155,  B)  branches  off  from  the  floor  of  the  chamber,  at  its  extreme  mesio- 
buccal angle,  and  the  entrance,  instead  of  being  funnel-shaped  and  easy 
of  access,  is  flattened  from  mesial  to  distal,  and  frequently  difficult  to 
enter.  This  flattened  form  continues  throughout  its  course,  which  for 
the  distance  of  1/8  of  an  inch  is  in  a  buccal  and  mesial  direction;  beyond 
this  point  it  is  usually  inclined  to  the  buccal,  until  the  upper  third  of  the 
root  is  reached,  where  it  turns  rather  abruptly  to  the  distal.  This  canal  is 
generally  a  trifle  shorter  than  the  lingual,  averaging  about  3/8  to  7/16  of 


D  e  F 

Fig.  155. — Pulp-cavities  of  Upper  First  Molar,  Slightly  Enlarged. 

an  inch.  The  distobuccal  canal  (Fig.  155,  C)  branches  off  from  the  floor 
of  the  chamber  at  the  extreme  distobuccal  angle.  In  those  teeth  which 
most  nearly  approach  the  quadrilateral  form,  the  entrance  to  this  canal 
will  be  farther  from  the  center  of  the  tooth,  the  molar  triangle  in  this 
instance  being  almost  an  equilateral.  It  sometimes  happens  that  the 
entrance  to  this  canal  is  directly  in  the  floor  of  the  pulp-chamber,  near  to, 
but  not  against,  its  buccodistal  angle.  The  entrance  is  usually  abrupt, 
seldom  being  funnel-shaped,  making  it  by  far  the  most  difficult  of  access. 
It  is  inclined  to  be  circular  in  form,  and  more  or  less  tortuous  in  its  course. 
Immediately  above  the  point  of  beginning  it  is  inclined  toward  the  buccal 
and  distal;  near  its  center  it  may  incline  slightly  to  the  mesial;  and  finally, 


208  ANATOMY 

at  its  upper  third,  turns  somewhat  abruptly  in  a  distobuccal  direction. 
This  canal  is  usually  the  shortest  of  the  three,  its  average  length  being 
about  3/8  of  an  inch. 

Figure  155  also  illustrates  a  number  of  transverse  sections  of  this 
tooth,  D  being  made  at  the  cervical  line,  looking  toward  the  crown,  E 
looking  toward  the  roots,  while  F  represents  a  transverse  section  at  a 
point  immediately  above  the  floor  of  the  pulp-chamber. 

Upper  Second  Molar. — In  many  respects  the  pulp-chamber  of 
this  tooth  is  similar  to  that  of  the  first  molar,  but  there  are  a  few  variations 
which  must  be  briefly  described.     First,  the  outline  of  the  tooth-crown 


9th  year  nth  year  13th  year  15th  year  18th  year 

Fig.  156. — Pulp-cavities  in  the  Upper  Second  Molar,  from  the  Ninth  to  the 
Eighteenth  Year. 

being  much  more  flattened  from  mesial  to  distal,  a  corresponding  varia- 
tion is  noted  in  the  form  of  the  pulp-chamber,  increasing  the  length  of 
the  mesial  side  of  the  molar  triangle,  and  decreasing  the  length  of  the 
buccal  and  distal  sides.  The  chamber  is  more  or  less  flattened  from 
mesial  to  distal,  making  it  somewhat  oblong  from  buccal  to  lingual. 
Second,  on  account  of  a  reduction  in  the  prominence  of  the  cusps,  the 
horns  of  the  cavity  are  usually  somewhat  less  pronounced  than  those  of 
the  first  molar.  Third,  the  floor  of  the  cavity  is  less  convex,  and  slopes 
more  gradually  toward  the  entrances  of  the  various  canals.  In  a  general 
way,  the  rules  given  for  ascertaining  the  approximate  location  of  the 
entrances  to  the  canals  in  the  first  molar  apply  to  this  tooth.  The  com- 
parative size  and  form  of  the  pulp-chamber  and  canals  during  the  develop- 
ment of  the  tooth  are  shown  in  figure  156,  extending  from  the  ninth  to 
the  sixteenth  or  eighteenth  year,  at  which  latter  period  the  crown  and 
roots  of  the  tooth  are  fully  calcified  externally. 

Upper  Third  Molar. — In  this  tooth  the  conditions  are  so  variable 
that  a  description  of  the  pulp-cavity  taken  from  a  single  tooth  would  be  in- 
sufficient.    In  the  majority  of  instances  the  outline  of  the  tooth-crown 


THE    PULP-CAVITIES    OF    THE    UPPER    TEETH  200. 

approaches  the  triangular  form,  and  in  consequence  the  pulp-chamber 
is  triangular  rather  than  quadrilateral  or  oblong.  The  mesial  border 
of  the  chamber  is  the  longest,  the  distal  next  in  length,  and  the  buccal 
the  shortest  of  the  three.  The  horns  are  generally  less  in  number  and 
much  less  pronounced  than  those  of  either  the  first  or  second  molars. 
The  floor  of  the  chamber  may  be  broken  by  irregularities  similar  to 
those  previously  described,  or  it  may  be  entirely  absent,  this  latter  con- 
dition occurring  when  the  tooth  has  but  a  single  root  accompanied  by  a 
single  canal.  The  various  stages  of  development  having  been  given  in 
connection  with  the  general  description  of  the  tooth,  no  attempt  will 


Fig.  157. — Longitudinal  Sections,  Upper  Third  Molar,  Slightly  Enlarged. 

be  made  to  describe  this  by  longitudinal  sections,  the  complications  in 
root-form  making  such  a  proceeding  impracticable.  Instead  of  so  doing, 
the  space  will  be  devoted  to  a  brief  description  of  the  variety  of  pulp- 
canals  found  in  this  tooth.  Probably  the  most  frequent  condition  is 
that  which  resembles  the  first  and  second  molars — i.e.,  three  canals 
branching  off  from  the  chamber  in  as  many  different  roots,  two  to  the 
buccal  and  one  to  the  lingual.  When  the  three  canals  exist,  the  entrances 
to  them  will  be  well  beyond  the  cervical  line,  where  they  will  be  found 
clustered  much  closer  together  than  those  of  the  first  and  second  molars, 
this  difference  in  their  location  being  so  marked  that  the  diagram  pre- 
viously given  cannot  be  depended  upon  in  an  attempt  to  locate  them. 
The  usual  course  of  these  canals  is  first  slightly  mesial,  then  distal,  and 
finally  in  a  distolingual  direction.  On  account  of  the  pulp-chamber 
extending  well  beyond  the  cervical  line,  the  canals  are  much  shorter  than 
those  of  the  first  or  second  molars,  their  average  length  being  less  than 
1/2  of  an  inch.  Another  form  frequently  met  with  is  that  of  the  flattened 
single  canal,  occurring  when  the  tooth  has  but  a  single  root,  which  shows 
14 


2IO  ANATOMY 

no  signs  of  trifucating  (Fig.  157,  A).  In  this  instance  the  pulp-chamber 
gradually  passes  into  the  canal,  and  the  chamber  is  without  a  floor.  Such 
a  canal  is  shaped  like  the  chamber  at  its  point  of  beginning;  but  as  it 
passes  toward  the  apex  it  becomes  flattened  in  the  direction  of  the  smallest 
diameter  of  the  root.  But  little  difficulty  is  experienced  in  entering 
such  a  canal,  and  usually  it  is  readily  followed  to  its  apex.  Another 
condition  frequently  met  with  in  the  single-rooted  third  molar  is  that 
of  one  or  more  canals  branching  off  from  the  floor  of  the  chamber,  their 
course  through  the  root-substance  being  without  regard  to  the  external 
contour  of  the  root  (Fig.  157,  B).  These  canals,  which  may  exist  to 
the  number  of  five  or  six,  are  usually  very  minute,  and  in  some  instances 
may  pass  from  the  floor  of  the  chamber  to  the  apex  of  the  root  almost 
in  a  direct  line,  and  end  in  distinct  foramina,  or  they  may  take  a  tortuous 
course,  and  when  near  the  apex  unite,  ending  in  a  single  foramen.  When 
the  tooth  is  provided  with  four,  five,  or  even  six  small  roots,  as  sometimes 
occurs,  each  root  will  be  traversed  by  a  minute  canal,  the  entrances  to 
these  being  variously  placed  about  the  floor  and  lateral  margins  of  the 
pulp-chamber  (Fig.  157,  C).  In  all  operations  upon  this  tooth  it  must 
be  recalled  that  it  is  the  last  to  be  calcified,  and  consequently  the  canals 
and  foramina  are  proportionately  larger  than  in  the  other  teeth;  at  the 
same  time,  it  possesses  one  advantage  over  the  others — i.e.,  (with  the 
single  exception  of  the  cuspid),  being  fully  calcified  at  or  about  the  time 
of  its  eruption. 


THE    PULP-CAVITIES    OF    THE    LOWER    TEETH  211 

PULP-CAVITIES  OF  THE  LOWER  TEETH. 

C  B  A 


Fig.  158. — Pulp-cavities  of  the  Lower  Incisor. 

The  outline  of  the  pulp-cavities  of  the  lower  teeth,  like  those  of  the 
upper,  corresponds  to  the  general  tooth  contour.  The  comparative  size 
of  the  cavity  at  various  stages  of  tooth  development  will  not  be  repeated 
in  this  description,  the  conditions  being  similar  to  those  in  the  upper 
teeth  (see  also  Development  of  the  Teeth). 

Lower  Incisors. — The  pulp-cavities  of  the  lower  central  and  lateral 
incisors  are  so  nearly  alike  that  a  single  description  will  answer  for 
both.  Figure  158,  A,  represents  a  labiolingual  section  of  a  lower  incisor, 
showing  the  most  frequent  form  of  the  pulp-cavity.  The  tooth  from 
which  the  section  was  prepared  was  one  about  middle  life,  the  cavity 
in  younger  teeth  being  proportionately  larger,  while  a  gradual  decrease 
in  diameter  would  be  noted  with  advancing  age.  There  is  no  mark 
of  distinction  between  the  pulp-chamber  and  canal,  so  that  an  imaginary 
separation  would  have  to  be  made  at  the  cervical  line,  or  slightly  below 
that  point.  Taken  in  its  entirety,  the  cavity  presents  the  form  of  a 
double  cone,  the  common  base  of  which  is  slightly  to  rootward  of  the 
cervical  line.  The  chamber  penetrates  the  crown  fully  half-way  to 
the  cutting-edge,  at  which  point  it  ends  in  a  thin,  fan-like  margin  (best 
observed  in  mesiodistal  section),  while  the  canal  gradually  decreases 
in  size  until  the  apical  foramen  is  reached.  Although  this  is  the  most 
common  form  of  the  pulp-cavity  in  the  lower  incisors,  it  is  by  no  means 
the  constant  condition.  The  tooth  is  not  infrequently  provided  with  a 
medium-sized  pulp-chamber,  which  extends  somewhat  below  the  cervical 


212 


ANATOMY 


line  beyond  which  point  it  branches  into  fine  canals,  which  are  continued 
separately  until  the  apical  third  of  the  roots  is  approached,  when  they 
again  unite,  and  finally  end  in  a  single  foramen.  Figure  158,  C,  repre- 
sents a  mesiodistal  section  of  a  young  lower  incisor,  in  which  the  three 
small  horns  of  the  pulp-chamber  are  apparent.  At  this  period  the  fan- 
shaped  extremity  of  the  pulp-chamber  occupies  about  one-half  of  the 
mesiodistal  diameter  of  the  crown,  and  the  horn-like  projections  extend 
well  toward  the  enamel  cap.  Figure  158,  B,  shows  the  average  size  and 
form  of  the  pulp-cavity  at  maturity,  by  a  mesiodistal  section  through  the 
long  axis  of  the  tooth.  In  this  it  will  be  observed  that  the  horns  of  the 
pulp-cavity  have  disappeared,  and  that  the  capacity  of  the  cavity  in 
general  is  much  reduced. 


Fig.  159. — Pulp-cavities  of  the  Lower  Cuspids. 

Lower  Cuspids. — The  pulp-chamber  and  canal  in  this  tooth,  while 
usually  conforming  to  the  general  contour  of  the  tooth,  are  frequently 
found  to  vary  greatly,  both  in  outline  and  in  size.  The  most  common 
form,  however,  is  that  shown  in  Fig.  159,  A,  a  labiolingual  section  of  an 
adult  tooth.  The  chamber  and  canal  have  no  line  of  demarcation,  and 
unite  at  a  common  base  considerably  below  the  cervical  line,  the  former 
penetrating  the  crown  of  the  tooth  to  a  point  about  midway  between  the 
cervical  line  and  the  summit  of  the  cusp,  at  which  point  it  ends  in  a  sharp, 
hair-like  projection.     Accompanying  this  common  form  there  is  much 


THE    PULP-CAVITIES    OF    THE    LOWER    TEETH 


213 


variation  in  size,  even  in  teeth  of  the  same  age.  Fig.  159,  B,  shows 
another  labiolingual  section  of  an  adult  lower  cuspid,  in  which  the  pulp- 
cavity  fails  to  accurately  follow  the  outline  of  the  tooth,  and  its  capacity 
is  much  less  than  that  shown  at  A.  The  root  of  this  tooth  is  in  most 
instances  circular,  in  which  case  the  canal  will  be  similarly  formed;  but 
occasionally  the  root  will  be  much  flattened  from  mesial  to  distal,  and 
as  a  result  of  this  the  canal  will  also  be  much  flattened.  The  canal  of 
this  tooth  is  seldom  divided.  In  Fig.  159,  C,  a  mesiodistal  section  of  a 
lower  cuspid  is  shown,  and  it  will  be  observed  that  the  fan-shaped  ex- 
tremity of  the  chamber  common  to  the  incisors  is  absent,  the  cavity  end- 
ing rather  abruptly,  or  by  a  fine  line  near  the  center  of  the  crown.  The 
same  illustration  also  shows  a  number  of  transverse  sections,  which  will 
give  an  idea  of  the  form  of  the  cavity  at  various  parts  of  the  tooth. 

Lower  Bicuspids. — The  pulp-cavities  of  these  teeth  may  be  best 
described  collectively,  thus  affording  an  opportunity  for  comparison. 
Unlike  the  upper  bicuspids,  it  is  seldom  that  the  canals  are  definitely 


Fig.  160. — Sections  of  Lower  Bicuspids. 


separated  from  the  chambers.  That  part  of  the  cavity  within  the  crown, 
however,  is  usually  quite  wide  from  buccal  to  lingual,  and  unites  with  the 
canal  by  a  long,  funnel-shaped  constriction.  The  center  of  the  pulp- 
chamber  may  be  considered  as  being  about  on  a  level  with  the  cervical  line. 
In  the  first  bicuspid  the  pulp-cavity  is  provided  with  a  single  horn,  which 
extends  with  more  or  less  prominence  in  the  direction  of  the  buccal  cusp. 
That  part  of  the  chamber  facing  the  lingual  cusp  is  usually  rounded  off. 


214  ANATOMY 

In  the  second  bicuspid  the  occlusal  wall  of  the  pulp-chamber  generally 
presents  a  different  form;  two  well-defined  horns  are  usually  present,  of 
which  the  buccal  is  the  longest;  or  the  chamber  may  be  prominently 
rounded  at  these  points.  The  pulp-chamber  of  the  second  bicuspid  is 
generally  larger  than  that  of  the  first.  The  canals  of  these  teeth  are 
usually  circular  throughout,  and  are  readily  penetrated  until  the  apical 
third  is  reached,  beyond  which  point  they  are  extremely  small.  In  some 
instances  the  canal  divides  near  the  center  of  the  root,  and  is  continued 
as  two  canals,  ending  in  distinct  foramina,  or,  after  separating,  they  may 
again  unite,  and  end  in  a  single  foramen.  In  Fig.  160  the  average  size 
and  form  of  the  canal  in  these  teeth  is  shown  by  a  number  of  mesiodistal 
and  transverse  sections. 


Fig.  161. — Sections  of  Lower  Molars,  Enlarged  about  One-third. 

Lower  Molars. — The  form  of  the  pulp-chambers  of  the  lower  molars 
corresponds  to  the  general  outline  of  the  crown,  and  the  form  of  the  root- 
canals  is  similar  to  the  general  contour  of  the  roots.  The  pulp-chambers 
approach  the  quadrilateral  form;  the  buccal  and  lingual  sides  are  some- 
what the  longest,  the  mesial  next  in  length,  and  the  distal,  usually  slightly 
rounded,  is  the  shortest.  The  occluding  wall  is  convex  rootward,  sloping 
i^  the  direction  of  the  various  cusps,  each  of  which  is  penetrated  by  a  horn, 
Like  the  horns  of  these  pulp-chambers  in  general,  the  extent  to  which  these 


PULP-CAVITIES    OF    THE    LOWER    TEETH  215 

penetrate  the  cusps  is  influenced  by  the  age,  type,  and  functional  activity  of 
the  organ.  The  floor  of  the  cavity  is  convex  in  the  direction  of  the  occlusal 
surface,  but  this  convexity  is  principally  from  mesial  to  distal.  From  the 
summit  of  this  convexity  the  floor  slopes  to  the  entrances  of  the  canals,  the 
opening  into  which  is  inclined  to  be  funnel-shaped  rather  than  abrupt. 
The  lateral  walls  of  the  chamber  are  much  inclined  to  follow  the  general 
contour  of  the  crown.  The  horns  of  the  pulp-chamber  are  usually  more  pro- 
nounced in  the  first  than  in  the  second  molar,  and  still  less  clearly  defined 
in  the  third  than  in  the  second.  The  roots  of  the  first  molar  being  some- 
what further  apart  than  those  of  the  second,  the  floor  of  the  chamber  in 
the  former  is  slightly  more  extensive  than  in  the  latter.  To  study  the 
pulp-cavities  of  these  teeth  a  longitudinal  section  should  be  made  through 
the  center  of  the  tooth  from  mesial  to  distal.  Fig.  161,  A,  shows  such  a 
dissection  through  the  first  molar,  and  illustrates  the  average  size  and 
form  of  the  chamber  and  canals  at  adult  age.  The  canals  join  the 
chamber  by  a  funnel-shaped  opening,  and  but  little  difficulty  will  be  found 
in  effecting  an  entrance,  but  to  follow  them  to  their  apices  will  be  more 
perplexing.  The  roots  of  this  tooth  being  much  flattened  from  mesial  to 
distal,  the  canals  are  also  flattened  in  this  direction,  but  broad  from  buccal 
to  lingual.  The  entrances  of  these  canals  may  be  found  at  the  extreme 
mesial  and  distal  margins  of  the  pulp-chamber,  and  usually  extend  from 
the  buccal  to  the  lingual  walls  of  the  cavity.  It  is  not  uncommon  for  the 
mesial  canal  to  divide  soon  after  leaving  the  chamber,  and  continue  as 
two  canals,  ending  in  separate  foramina  (Fig.  161,  B).  This  condition 
is  seldom  present  in  the  distal  canal,  which  is  usually  straight  from  its 
mouth  to  the  apical  foramen.  The  capacity  of  the  pulp-chamber  is 
usually  a  trifle  less  than  that  of  the  first  molar,  and  the  entrances  to  the 
canals  are  somewhat  nearer  together.  In  other  respects  the  cavity  is  simi- 
lar to  that  of  the  first  molar.  Fig.  161  also  shows  a  number  of  transverse 
sections  through  a  lower  molar,  and  gives  an  idea  of  the  size  of  the  canals 
at  various  parts  of  the  roots.  In  some  instances  the  roots  of  the  second 
molar  coalesce,  in  which  case  a  single  root-canal  may  be  present.  In 
the  third  molar  the  most  common  form  of  the  pulp-cavity  is  one  similar 
to  that  of  the  first,  but  both  the  chamber  and  canals  are  smaller.  Unlike 
the  pulp-cavity  of  the  corresponding  upper  tooth,  this  tooth  is  not  sub- 
ject to  so  much  variation,  although  it  is  sometimes  found  with  a  single 
root  traversed  by  a  single  canal,  which  may  be  accompanied  by  a  rather 
large  pulp-chamber. 


CHAPTER  XL 

The  Deciduous  Teeth,  Their  Arrangement,  Occlusion,  Etc. ;  Their 
Calcification,  Eruption,  Decalcification,  Shedding  Process, 
and  Average  Measurements;  Their  Surfaces,  Grooves,  Fossae, 
Ridges,  Sulci,  and  Pulp-Cavities. 

THE  DECIDUOUS  TEETH. 


Central  Incisor 


Lateral  Incisor 


Cuspid 


First  Molar 


Second  Molar 


Uppei 


Lower 


Central  Incisor  Lateral  Incisor  Cuspid  First  Molar  Second  Molar 

Fig.  162. — The  Deciduous  Teeth,  Upper  and  Lower,  from  the  Left  Side  of  the  Mouth. 

As  implied  by  the  word  deciduous,  these  teeth  are  temporary  in  their 
nature,  and,  after  subserving  the  purposes  of  early  childhood,  are  thrown 
off  by  an  operation  of  the  economy  to  give  place  to  the  permanent  organs. 
The  shedding  process  takes  place  in  the  incisors  between  the  seventh 
and  eighth  years,  in  the  molars  from  the  tenth  to  the  eleventh  years,  and 
in  the  cuspids  about  the  twelfth  year.     This  shedding  process,  however, 

216 


THE    DECIDUOUS    TEETH  217 

does  not  indicate  the  period  at  which  the  degeneracy  of  the  tooth  begins, 
for,  in  a  year  or  two  after  the  root  is  completely  formed  and  the  apical 
foramen  established,  decalcification  begins  at  the  apical  extremity  and 
continues  in  the  direction  of  the  crown  until  absorption  of  the  entire  root 
has  taken  place  and  the  crown  is  lost  from  lack  of  support.  Decalcifi- 
cation in  the  incisors  begins  between  the  fourth  and  fifth  years,  in  the 
molars  from  the  seventh  to  the  eighth  years,  and  in  the  cuspids  about 
the  ninth  year. 

The  deciduous  teeth  are  twenty  in  number,  ten  in  each  jaw,  and  may 
be  classified  as  follows:  Four  incisors,  two  cuspids,  and  four  molars. 
The  incisors,  central  and  lateral,  occupy  the  central  portion  of  the  arch, 
are  placed  two  upon  each  side  of  the  median  line,  and  are  succeeded  by 
the  four  permanent  incisors,  which  finally  occupy  the  same  position. 
The  cuspids  are  located  immediately  to  the  distal  of  the  lateral  incisors, 
and  are  displaced  by  the  permanent  cuspids.  The  first  and  second  molars 
come  next  in  the  arch,  but,  unlike  the  anterior  teeth,  are  followed  by  per- 
manent successors  of  another  class,  the  first  and  second  bicuspids,  the 
permanent  molars  erupting  posteriorly  to  these  as  the  jaw  increases  in 
length. 

In  general  the  deciduous  teeth  resemble  their  permanent  successors, 
yet  there  are  a  number  of  minor  differences  which  will  require  a  compara- 
tive description.  Both  the  crowns  and  the  roots  are  much  smaller  in 
every  direction  than  those  of  the  permanent  teeth,  but  the  diameter  of 
the  crowns  is  proportionately  greater  than  that  of  the  roots,  while  the 
roots  are  proportionately  longer.  The  fact  that  the  roots  are  smaller  in 
proportion  than  the  crowns  is  productive  of  a  neck  much  more  constricted. 
The  roots  of  the  deciduous  teeth  are  the  same  in  number  as  those  of  the 
corresponding  permanent  teeth,  the  incisors  and  cuspids  being  provided 
with  one,  the  upper  molars  with  three,  and  the  lower  molars  with  two. 


2l8 


ANATOMY 


THE  OCCLUSION  OF  THE  DECIDUOUS  TEETH. 

Cuspid  Incisors 


First  Molar 
Second  Molar 


Permanent 
First  Molar 


Fig.  163.—  The  upper  Dental  Arch  about  the  Seventh  Year. 

The  arrangement  of  the  deciduous  teeth  in  the  jaws  is  similar  to  that 
of  the  permanent  organs,  the  upper  teeth  describing  the  segment  of  a 
larger  circle  than  the  lower,  in  consequence  of  which  the  upper  teeth 
close  over  or  outside  of  the  lower.  The  character  of  the  occlusion  in  the 
deciduous  teeth  is  not  subject  to  so  much  variation  as  that  found  in  con- 
nection with  the  permanent  set,  this  being  accounted  for  by  the  more 
constant  form  in  the  crowns  of  the  former.  The  relations  existing  between 
the  upper  and  lower  deciduous  teeth  when  in  contact  is  such  that  each 
tooth,  with  the  exception  of  the  lower  central  incisor  and  the  upper  second 
molar,  occludes  with  two  teeth  of  the  opposite  jaw,  the  upper  central 
incisor  being  opposed  by  the  entire  cutting-edge  of  the  lower  central  and 
the  mesial  third  of  the  lower  lateral;  the  upper  lateral  coming  in  contact 
with  the  remaining  two-thirds  of  the  lower  lateral  and  a  portion  of  the 
mesial  half  of  the  lower  cuspid,  this  arrangement  continuing  throughout 
the  series.  The  foregoing  description  of  the  occlusion  of  the  deciduous 
teeth  is  applicable  to  but  a  small  part  of  their  transitory  existence.  Soon 
after  they  are  fully  erupted  and  have  assumed  their  respective  posi- 
tions in  the  arch,  the  increase  in  the  size  of  the  bone  is  sufficient  to  create 
a  slight  space  between  the  teeth,  which  condition  is  soon  followed  by  a 
greater  separation  through  the  protrusion  of  the  anterior  teeth,  caused  by 
the  growth  and  approach  of  the  permanent  teeth  from  behind. 

The  calcification  of  the  deciduous  teeth  is  similar  to  that  of  the  per- 
manent, the  process  in  the  incisors  and  cuspids  beginning  along  the  cut- 


THE   DECIDUOUS   TEETH   IN   DETAIL 


219 


ting-edges  in  three  distinct  lobes,  while  in  the  molars  a  center  of  calcifi- 
cation is  provided  for  each  cusp  (see  Development  of  the  Teeth). 

THE  DECIDUOUS  TEETH  IN  DETAIL. 

UPPER  CENTRAL  INCISOR 


Eighteenth  Month 
after  Birth 


Sixth  Month  after 
Birth 


Fortieth  Week 


Twentieth  Week 


Fourth  Year 


Fifth  Year 


Seventh  Year 


Fig.  164. 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Seventeenth  to  Eighteenth  Month  after  Birth. 
Erupts,  Sixth  to  Eighth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fourth]Year. 

Shedding  Process  Takes  Place,  about  the  Seventh  Year. 
Average  Length  of  Crown,  .23. 

Average  Length  of  Root,  .39. 

Average  Length  over  All,  .62. 

This  tooth,  as  well  as  all  of  the  deciduous  teeth,  presents  for  examina- 
tion numerous  surfaces,  margins,  and  angles,  these  being  the  same  in 
name  and  location  as  those  of  the  permanent  teeth. 

The  Labial  Surface  of  the  Crown  (Fig.  164). — This  surface  is  smooth 
and  generally  convex,  but  with  an  inclination  to  flatness  near  the  incisive 
margin.  The  mesial  margin  is  slightly  convex  in  the  direction  of  the 
length  of  the  tooth,  and  rounded  from  labial  to  lingual.  The  distal  mar- 
gin is  decidedly  convex  from  the  cutting-edge  to  the  cervical  line,  in  many 
instances  forming  almost  a  complete  semicircle,  which  is  usually  at  the 
expense  of  the  distal  angle  of  the  crown.  The  cervical  margin  is  deeply 
concave  in  the  direction  of  the  root,  and  the  incisive  margin  is  straight  over 
its  central  portion  and  rounded  or  angular  at  its  extremities.  The  labial 
grooves  are  seldom  so  well  defined  at  those  upon  the  permanent  incisors. 

The  Lingual  Surface  of  the  Crown. — In  some  instances  this 
surface  is  smooth  and  concave  near  the  cutting-edge  and  convex  over  the 
cervical  portion,  with  the  marginal  ridges  well  defined.  In  other  cases  it  is 
concave  from  the  cutting-edge  to  the  cervical  ridge,  being  provided  with  a 
longitudinal  ridge  in  the  center,  a  slight  depression  upon  either  side,  and 


2  20 


ANATOMY 


marginal  ridges  poorly  defined.  In  the  former  instance  the  lingual 
fossa  is  present;  in  the  latter  it  is  absent.  The  mesial  and  distal  sur- 
faces of  the  crown  are  both  smooth  and  convex,  the 
former  being  inclined  to  flatness  over  its  cervical 
third — a  condition  which  is  seldom  present  in  the 
latter.  The  mesial  angle  is  alone  well  defined,  the 
cutting-edge  passing  into  the  distal  surface  with  a 
long,  gradual  sweep,  thus  in  a  measure  destroying 
the  distal  angle.  The  neck  of  the  tooth  is  marked 
by  a  decided  constriction,  which  is  principally  pro- 
duced at  the  expense  of  the  crown  alone.  The 
root  of  the  tooth,  when  compared  with  the  root  of 
the  permanent  central  incisor,  is  much  longer  in 
proportion  to  the  length  of  the  crown.  In  some 
instances  it  is  flattened  from  mesial  to  distal, 
these  two  sides  converging  as  they  pass  to  the  lingual;  in  others  it  is  flat- 
tened from  labial  to  lingual.  Generally  speaking,  it  is  a  single  root, 
but  is  occasionally  provided  with  a  slight  mesial  curve  near  its  apical 
third,  and  it  is  sometimes  curved  slightly  from  labial  to  lingual. 


Fig.  165. 


UPPER  LATERAL  INCISOR. 


Sixteenth  Month 
after  Birth 


Fortieth  Week 


Twentieth  Week 


Fifth  Year 


Seventh  Year 


Eighth  Year.  j(* 


Fig.  166. 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Fourteenth  to  Sixteenth  Month  after  Birth. 
Erupts,  Seventh  to  Ninth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fifth  Year. 

Shedding  Process  Takes  Place  about  the  Eighth  Year. 
Average  Length  of  Crown,  .25. 

Average  Length  of  Root,  .45. 

Average  Length  over  All,  .70. 


The  various  surfaces  of  this  tooth  so  closely  resemble  those  of  the  cen- 
tral incisor  that  a  separate  description  will  be  unnecessary;  in  a  general 


THE    DECIDUOUS    TEETH   IN    DETAIL 


221 


way,  however,  there  are  a  few  minor  points  of 
difference.  The  tooth  is  smaller  in  every  direc- 
tion excepting  in  its  length,  which  is  generally 
equal  to  and  frequently  greater  than  that  of  the 
central  incisor.  The  diameter  of  the  root  is  but 
little  less  than  that  of  the  central,  while  the 
mesiodistal  measurement  of  the  crown  is  about 
one-third  less,  in  consequence  of  which  the  neck 
of  the  tooth  is  not  so  well  defined.  The  angles  of 
the  crown  are  more  rounded  than  those  of  the 
central  incisors. 


Fig.     167. — Upper    Lateral 
Incisor,  Labial  Surface. 


UPPER  CUSPID. 


Second  Year 


Sixth  Month 
after  Birth 


Ninth  Year 


Tenth  Year 


Twelfth  Year 


Birth 


Thirtieth  Week 
Embryo 


Fig.  168. 

Calcification  Begins,  about  the  Fifth  Fetal  Month. 

Calcification  Completed,  about  Two  Years  after  Birth. 

Erupts,  Seventeenth  to  Eighteenth  Month  after  Birth 
Decalcification  Begins,  about  the  Ninth  Year. 

Shedding  Process  Takes  Place,  about  the  Twelfth  Year. 
Average  Length  of  Crown,  .25. 

Average  Length  of  Root,  .53. 

Average  Length  over  All,  .78. 


Like  the  permanent  cuspid,  the  general  contour  of  this  tooth  is  that  of 
a  double  cone,  the  lines  of  which  are  somewhat  broken.  The  greatest 
mesiodistal  extent  of  the  crown  is  from  angle  to  angle,  and  this  measure- 
ment about  corresponds  with  the  width  of  the  crown  of  the  central  incisor. 

The  Labial  Surface  of  the  Crown  (Fig.  168). — This  surface  is 
strongly  convex  from  mesial  to  distal,  and  slightly  so  from  the  cutting-edge 
to  the  cervical  line.     It  is  bounded  by  five  margins:  mesial,  distal,  cervical, 


222 


AN  \h>MY 


mesio-incisive,  and  disto-incisive.  The  mesial  and  distal  margins  are 
rounded  and  smooth,  the  cervical  well  outlined  by  the  cervical  line  and 
base  of  the  cervical  ridge,  while  the  two  incisive  margins  are  formed  by 
the  mesial  and  distal  cutting-edges.  The  labial  grooves  are  thrown  well 
toward  the  lateral  margins,  and  are  usually  more  distinct  than  those 
upon  the  incisors.     The  labial  ridge  is  prominent. 

The  Lingual  Surface  of  the  Crown. — This 
surface  is  generally  divided  into  two  portions  by 
the  lingual  ridge,  which  extends  from  the  sum- 
mit of  the  cusp  to  the  base  of  the  cervical  ridge. 
On  either  side  of  this  ridge  are  the  lingual 
grooves,  but  which  appear  more  in  the  form  of 
small  fossae.  The  marginal  ridges  are  fairly  well 
defined. 

The  Mesial  and  Distal  Surfaces  of  the 
Crown. — The  extent  of  these  two  surfaces  is  fre- 
quently much  interfered  with  by  the  slope  of  the 
mesial  and  distal  cutting-edges,  which  may  be  so 
long  that  the  angles  of  the  crown  are  forced 
well  toward  the  cervical  line,  in  some  instances  almost  obliterating 
these  two  surfaces.  When  the  cutting-edges  are  shorter,  these  sur- 
faces present  a  marked  general  convexity.  While  the  summit  of  the  cusp 
will  always  be  found  to  be  in  a  direct  line  with  the  long  axis  of  the  tooth, 
there  is  in  nearly  every  instance  a  difference  in  the  length  of  the  cutting- 
edges,  and.  unlike  the  cutting-edges  of  the  permanent  cuspid,  the  mesial 
is  usually  the  longer.  The  neck  of  the  tooth  is  much  constricted  and 
the  root  straight  and  conic. 


Fig.  169. — Upper  Cuspid. 
Labial  Surface. 


THE    DECIDUOUS    TEETH    IX    DETAIL 

THE  ITPER  MOLARS. 


223 


Upper  First  Molar. 


.  wenty-two 

Months  Old 


Or.e  Year  Old 


Forty  Wee- 


Fig.  170. 
Calcification  Begins,  about  the  Fifth  Fetal  Month. 

calcification  completed,  eighteenth  to  twentieth  month  after  blrth. 
Erupts.  Fourteenth  to  Fifteexth  Moxth  after  Birth. 
Decalcification  Begixs.  Sixth  to  Se\"exth  Ye.ar. 

Sheddlxg  Process  Takes  Place,  .about  the  Texth  Ye.ar. 
A\"erage  Lexgth  of  Crowx.  .20. 

Average  Length  of  Root,  .39. 

Average  Lexgth  over  All.  .59. 

The  contour  and  lobate  construction  of  the  crown  of  this  tooth  is 
peculiar  to  itself,  being  dissimilar  to  any  other  class  of  teeth  in  the  mouth. 
Calcification  takes  place  from  three  centers,  two  for  the  buccal  and  one 
for  the  lingual  half  of  the  crown.  The  general  form  of  the  crown  may 
best  be  studied  by  an  examination  of  the  occlusal  surface. 

The  Occlusal  Surface  of  the  Crown. — The  outlines  represented  are 
those  of  an  irregular  quadrilateral,  of  which  the  buccal  and  mesial  sides 
are  the  longest.  The  angles  of  the  quadrilateral  are  somewhat  variable. 
the  mesiobuccal  being  acute,  the  mesiolingual  obtuse,  while  the  two 
distal  angles  are  rounded  right  angles.  The  surface  is  surmounted  by 
three  cusps,  a  mesiobuccal.  a  distobuccal,  and  a  lingual.  These  various 
cusps  are  separated  from  one  another  by  three  developmental  grooves — the 
mesial,  the  distal,  and  the  buccal.  The  marginal  ridges  are  sharp  and 
well  defined,  this  being  particularly  true  of  the  buccal  and  lingual,  which 
resemble  cutting-edges.  The  mesio-marginal  ridge  begins  at  the 
mesiobuccal  angle,  and.  after  making  a  Long  distal  curve,  ends  in  the 
mesial  incline  of  the  lingual  cusp.  The  center  of  the  surface  is  deeply 
and  irregularly  concave,  producing  the  central  fossa,  and  descending 
from  the  various  ridges  and  cusps  surrounding  it  are  numerous  supple- 
mental grooves  and  ridges.  The  various  developmental  grooves  are  not 
inclined  to  cross  the  marginal  ridges,  although  in  some  instances  one  or 
two  mav  be  found  to  do  so 


224 


ANATOMY 


The  Buccal  Surface  of  the  Crown  (Fig.  170). — This  surface  is 
generally  smooth  and  convex,  with  an  excessively  developed  cervical  ridge, 
which  is  particularly  prominent  at  its  mesial  extremity.  The  buccal 
groove  is  in  the  form  of  a  slight  depression,  and  the  buccal  ridges,  common 
to  all  molars,  are  scarcely  to  be  observed.  The  mesial,  occlusal,  and  cer- 
vical margins  are  distinctly  outlined,  while  the  distal  margin  is  obliterated 
by  the  gradual  passing  of  this  surface  into  the  distal  surface. 

The  Lingual  Surface  of  the  Crown. — This  surface  is  circular  in 
outline,  decidedly  convex  and  smooth,  and  is  seldom  broken  by  grooves 

and  ridges.  It  is  most  prominent 
near  the  center,  from  which  point 
it  slopes  in  every  direction.  The 
cervical  ridge  is  not  so  pronounced 
as  that  of  the  buccal  surface,  but 
there  is  a  sudden  rounding  of  the 
surface  in  a  cervical  direction  to 
meet  the  lingual  root. 

The  Mesial  Surface  of  the 
Crown. — This  surface  is  probably 
more  extensive  than  any  of  the 
others;  it  is  inclined  to  flatness, 
with  a  slight  conic  convexity  over 
its  occlusal  third,  and  a  slight  con- 
cavity near  the  cervix.  The  buc- 
colingual  measurement  of  the  sur- 
face is  nearly  twice  as  great  as  that  from  the  occlusal  margin  to  the 
cervical  line.  It  is  much  more  prominent  near  the  occlusal  margin,  so 
that  a  V-shaped  space  usually  exists  between  it  and  the  distal  surface  of 
the  cuspid. 

The  Distal  Surface  of  the  Crown. — The  extent  of  this  surface 
is  much  less  than  that  of  the  mesial;  it  presents  a  general  convexity,  and 
is  seldom  broken  by  grooves  or  ridges,  although  occasionally  the  distal 
groove  crosses  its  occlusal  margin.  Like  the  deciduous  teeth  previously 
described,  the  neck  of  the  tooth  is  marked  by  a  decided  and  abrupt  con- 
striction, this  form  appearing  to  arise  from  the  heavy  enamel  folds  which 
are  present  near  the  cervical  line,  rather  than  from  any  marked  con- 
striction in  the  base  of  the  roots  themselves. 

The  roots  of  the  tooth  are  three  in  number — a  mesiobuccal,  a  disto- 
buccal,  and  a  lingual;  of  these,  the  latter  is  usually  the  largest  and  longest. 
The  two  buccal  roots  are  much  flattened  from  mesial  to  distal,  while  the 


Fig.  171. — Occlusal  Surfaces  of  the 
Deciduous  Molars. 


THE    DECIDUOUS    TEETH   IN    DETAIL 


225 


lingual  is  compressed  in  the  opposite  direction.  The  apical  ends  of  the 
roots  are  much  separated  from  one  another,  the  triangle  which  these 
points  form  being  almost  twice  the  size  of  the  triangle  formed  by  the  base 
of  the  roots.     The  apical  ends  are  usually  provided  with  a  central  curve. 

Upper  Second  Molar. 


One  Year  Old 


Eight  Years  m 


Ten  Years 


Eleven  Years 


Fig.  172. 

Calcification-  Begins,  between  the  Fifth  and  Sixth  Fetal  Months. 

Calcification  Completed,  Twentieth  to  Twenty-second  Month  after  Birth. 
Erupted;  Eighteenth  to  Twenty-fourth  Month  after  Birth. 
Decalcification  Begins,  Seventh  to  Eighth  Year. 

Shedding  Process  Takes  Place,  Eleventh  to  Twelfth  Year. 
Average  Length  of  Crown,  .22. 

Average  Length  of  Root,  .46. 

Average  Length  over  All,  .68. 


The  most  remarkable  feature  about  the  crown  of  this  tooth  is  its  close 
resemblance  to  the  crown  of  the  upper  permanent  first  molar.     The 
various  surfaces   are   almost  identical,   the  de- 
velopmental process,  and  consequently  the  cusp- 
formation,  is  the  same,  the  marginal  and  other 
ridges   common   to   the   occlusal   surface   corre- 
spond, and  both  the  central  and  d,istal  fossae  are 
present,  together  with  the  various  developmental 
grooves.     A  description  of  the  crown  will,  there- 
fore, be  unnecessary;  suffice  it  to  say  that  it  is 
much  smaller  in  every  direction  and  is  some- 
what more  constricted  at  the  neck.     The  roots 
are  the  same  in  name  and  number  as  those  of 
the  first  permanent  molar,  but  they  are  more 
widely  separated  at  their  apical  extremities.     In  general  form  they  are 
smaller  than  those  of  the  upper  first  deciduous  molar. 
15 


Fig.  173. — Upper  Sec- 
ond Molar,  Buccal 
Surface. 


226  ANATOMY 


THE  LOWER  DECIDUOUS  TEETH. 

A  description  in  detail  of  the  lower  incisors  and  cuspids  would  practi- 
cally be  a  repetition  of  that  given  of  the  corresponding  upper  teeth,  and 
for  that  reason  will  be  passed  with  a  limited  reference  to  each.  The 
lower  molars  being  in  many  respects  unlike  the  upper,  they  will  require 
a  separate  description. 

Lower  Central  Incisor  (Fig.  174). 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Sixteenth  to  Eighteenth  Month  after  Birth. 
Erupted,  Sixth  to  Eighth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fourth  Year. 

Shedding  Process  Takes  Place,  about  the  Seventh  Year. 
Average  Length  of  Crown,  .19. 

Average  Length  of  Root,  .35. 

Average  Length  over  All,  .54. 

This  is  the  smallest  of  the  lower  teeth,  in  this 
respect  being  at  variance  to  the  upper  central,  which 
is  larger  than  the  lateral.  The  mesiodistal  diameter 
of  the  crown  is  but  little  less  than  that  from  the 
cutting-edge  to  the  cervical  line.  The  mesial  and 
distal  angles  are  similar,  both  being  pointed  and 
square.  The  cervical  ridge  is  quite  pronounced  and 
the  neck  much  constricted. 

The  root  is  usually  straight  and  tapers  gradually 
Fig.  174-—         from  base  to  apex.     It  is  broader  on  the  labial  than 

.Lower  Central  1 

incisor,  Labial        on  the  lingual  side,  and  the  mesial  and  distal  sides 
are  but  little  flattened. 


Surface. 


Lower  Lateral  Incisor  (Fig.  175). 

Calcification  Begins,  about  the  Fourth  Fetal  Month. 

Calcification  Completed,  Twelfth  to  Fourteenth  Month  after  Birth. 
Erupted,  Seventh  to  Ninth  Month  after  Birth. 
Decalcification  Begins,  about  the  Fifth  Year. 

Shedding  Process  Takes  Place  about  the  Eighth  Year. 
Average  Length  of  Crown,  .19. 

Average  Length  of  Root,  .39. 

Average  Length  over  All,  .58. 

This  tooth  is  larger  than  the  central  incisor,  and  closely  resembles  the 
upper  lateral  both  in  size  and  form.     The  crown  is  more  rounded  in  its 


THE    DECIDUOUS    TEETH   IN    DETAIL 


227 


nature  than  that  of  the  central,  forming  a  greater  general  convexity  to 
the  labial  surface,  and  less  concavity  to  the  lingual.  The  mesial  angle  of 
the  crown  is  fairly  well  denned,  while  the  distal  is 
usually  much  rounded  by  a  long,  circular  sweep  of 
the  cutting-edge  to  meet  the  distal  surface. 

The  mesial  surface  of  the  crown  is  flattened  and 
somewhat  prominent  at  the  angle,  while  the  distal 
surface  is  strongly  convex.  The  labial  grooves  are 
but  slightly  visible,  while  the  corresponding  lingual 
grooves  are  quite  pronounced.  The  neck  of  the  tooth 
is  even  more  marked  than  that  of  the  lower  central 
incisor..  The  root  is  long  and  tapering,  slightly  flat- 
tened from  mesial  to  distal,  with  a  decided  longitudinal 
groove  on  both  the  mesial  and  distal  sides.  The 
labial  and  lingual  sides  are  rounded,  and  there  is  an 
inclination  to  crookedness,  which  is  usually  from  mesial  to  distal 


Fig.  175  — 
Lower  Lateral 
Incisor,  Labial 
Surface. 


Lower  Cuspid  (Fig.  176). 


Calcification  Begins,  about  the  Fifth  Fetal  Month. 

Calcification  Completed,  about  Two  Years  after  Birth. 

Erupted,  Seventeenth  to  Eighteenth  Month  after  Birth. 
Decalcification  Begins,  about  the  Ninth  Year. 

Shedding  Process  Takes  Place,  about  the  Twelfth  Year. 
Average  Length  of  Crown,  .23. 

Average  Length  of  Root,  .45. 

Average  Length  over  All,  .68. 


Fig.  176.  — 
Lower  Cuspid, 
Labial  Surface. 


The  principal  variations  between  this  tooth  and 
the  upper  cuspid  are  observed  in  the  diminished 
mesiodistal  measurement  of  the  crown,  together  with 
it  being  somewhat  less  angular  in  outline.  The 
ridges  and  grooves  common  to  the  various  surfaces 
are  not  so  marked  as  those  of  the  upper  cuspid,  re- 
sulting in  a  smoothly  formed  crown  throughout.  The 
root  is  larger  in  proportion  to  the  size  of  the  crown 
than  that  of  its  upper  opponent,  thus  producing  a 
neck  much  less  constricted.  It  is  usually  straight,  or 
provided  with  a  slight  distal  inclination  near  its  apical 
extremity,  and  much  flattened  from  mesial  to  distal, 
these  two  sides  converging  to  the  lingual,  forming  a 
rounded  triangular  outline. 


2  28  ANATOMY 

Lower  First  Molar  (Fig.  177). 

Calcification  Begins,  about  the  Fifth  Fetal  Month. 

Calcification  Completed,  Eighteenth  to  Twentieth  Month  after  Birth. 
Erupted,  Fourteenth  to  Fifteenth  Month  after  Birth. 
Decalcification  Begins,  Sixth  to  Seventh  Year. 

Shedding  Process  Begins,  about  the  Tenth  Year. 
Average  Length  of  Crown,  .24. 

Average  Length  of  Root,  .38. 

Average  Length  over  All,  .62. 

Upon  making  an  examination  of  the  occlusal  surface  of  this  tooth  it 
will  be  observed  that  the  crown  is  made  up  of  four  irregularly  formed  lobes, 
separated  from  one  another  by  four  well-defined  grooves.     Each  lobe  is 

provided  with  a  cusp,  more  or  less  prominently 
developed.  Between  the  various  cusps  are  two 
fossae — one  occupying  the  distal  two-thirds  of  the 
surface  (the  distal  fossa)  and  the  other  the  re- 
maining or  mesial  third  (the  mesial  fossa). 
The  outline  of  this  surface,  which  represents 
the  contour  of  the  crown  in  general,  is  that  of 
an  oblong  square,  with  its  angles  more  or  less 
Fig.    177.— -Lower  First  r0unded,   and  having  a  slight  variation  in  its 

Molar,  Buccal  Surface.  _  °  ° 

parallel  lines.  Each  lobe  denotes  a  separate 
center  of  calcification,  and  the  four  giooves  the  lines  of  union  between 
the  various  parts. 

The  mesiobuccal  lobe  is  somewhat  irregular  in  contour  and  is  fre- 
quently the  largest  of  the  four.  It  assists  in  forming  the  mesiobuccal 
angle  of  the  crown  and  the  greater  part  of  the  mesial  fossa.  Descending 
from  this  cusp  to  the  lingual  is  a  pronounced  triangular  ridge,  which  is 
made  continuous  by  uniting  with  a  similar  ridge  from  the  corresponding 
lingual  cusp.  By  this  union  a  transverse  ridge  is  established,  separating 
the  mesial  from  the  distal  fossa.  The  central  boundary  of  this  lobe 
is  formed  by  the  mesial  groove,  which  arises  from  the  distal  fossa,  passes 
over  the  transverse  ridge  to  the  mesial  fossa,  from  which  it  continues  to 
the  lingual,  and  by  the  buccal  groove,  which  branches  off  from  the 
mesial  somewhat  to  the  distal  of  the  transverse  ridge,  passing  over  the 
buccomarginal  ridge  to  the  buccal  surface. 

The  Distobuccal  Lobe. — This  cusp  is  generally  smaller  than  the 
mesiobuccal,  and  is  more  pointed  and  more  regular  in  outline.  It  assists 
in  forming  the  distobuccal  angle  of  the  crown,  and  by  its  central  incline 
forms  about  one-third  of  the  distal  fossa.  Its  boundaries  are  formed  by 
the  buccal,  mesial  and  distal  grooves,  the  latter  beginning  in  the  distal 
fossa,  and  passing  over  the  distomarginal  ridge  to  the  distal  surface. 


THE    DECIDUOUS    TEETH    IN    DETAIL  229 

The  Mesiolingual  Lobe. — In  the  recently  erupted  tooth  the  summit 
of  this  cusp  is  long  and  pointed,  and  frequently  remains  the  most  pro- 
nounced of  the  four.  It  is  triangular  in  outline,  and,  as  above  referred  to, 
furnished  a  triangular  ridge,  which,  by  uniting  with  a  like  ridge  from  the 
mesiobuccal  cusp,  forms  the  transverse  ridge.  By  its  central  incline  it 
assists  in  forming  the  mesial  fossa.  Its  boundaries  are  formed  by  the 
mesial  groove  and  the  lingual  groove,  the  latter  arising  near  the  center  of  the 
distal  fossa,  passing  to,  and  sometimes  crossing,  the  linguomarginal  ridge. 

The  Distolingual  Lobe. — This  is  usually  the  smallest  of  the  four. 
It  is  inclined  to  be  rounded,  rather  than  angular,  and  in  some  instances 
is  poorly  developed.  It  assists  in  forming  the  distolingual  angle  of  the 
crown,  as  well  as  a  portion  of  the  distal  fossa.  Its  central  boundaries  are 
formed  by  the  lingual  and  distal  grooves. 

The  marginal  ridges  of  the  surface  are  abruptly  but  irregularly 
formed,  ascending  and  descending  the  various  cusps  in  a  manner  similar 
to  those  previously  described. 

The  Buccal  Surface  of  the  Crown  (Fig.  177). — This  surface  is 
smooth  and  generally  convex,  with  a  mesiodistal  measurement  about 
twice  as  great  as  that  from  the  cervical  line  to  the  occlusal  margin. 
The  surface  is  most  prominent  over  its  cervical  third,  forming  a  well- 
rounded  and  bold  cervical  ridge,  a  feature  strongly  characteristic  of  this 
tooth.  The  distal  center  of  the  surface  is  broken  by  the  buccal  groove, 
which  usually  ends  near  the  center  in  a  shallow  depression  or  pit. 

The  Lingual  Surface  of  the  Crown. — This  surface  is  much  less 
extensive  than  the  buccal.  It  is  smooth  and  convex  throughout, 
and  is  broken  near  its  distal  center  by  the  lingual  groove,  which  gradually 
disappears  as  it  passes  rootward.  The  cervical  ridge  is  not  so  prominent 
as  that  of  the  buccal  surface. 

The  Mesial  and  Distal  Surfaces  of  the  Crown. — These  are  slightly 
convex  in  every  direction,  the  former  passing,  by  a  gradual  sweep,  into 
the  lingual  surface,  destroying  the  angularity  of  the  crown  at  this 
point,  while  the  latter  passes  more  abruptly,  forming  an  acute  angle. 
These  surfaces  are  both  prominent  near  the  occlusal  margin,  making  the 
point  of  contact  with  adjoining  teeth  near  the  surface.  The  bold  cer- 
vical ridge  of  the  buccal  surface  is  discontinued  or  greatly  diminished 
upon  these  surfaces,  both  of  which  are  inclined  to  pass  very  gradually 
into  the  base  of  the  roots. 

The  roots  of  this  tooth  are  the  same  in  name,  position,  and  number 
as  those  of  the  lower  permanent  molars.  They  are  much  flattened  from 
mesial  to  distal,  the  center  of  their  flattened  sides  being  further  com- 


23O  ANATOMY 

pressed  by  a  deep  longitudinal  groove,  which  extends  from  the  base  to 
the  apex  of  each  root.  In  passing  from  the  base  of  the  roots  to  their 
apices  they  become  more  widely  separated,  until  these  extremities  are 
much  wider  apart,  proportionately,  than  those  of  the  permanent  molars. 

Lower  Second  Molar  (Fig.  178). 

Calcification  Begins,  between  the  Fifth  and  Sixth  Fetal  Months. 

Calcification  Completed,  Twentieth  to  Twenty-second  Month  after  Birth. 
Erupted,  Eighteenth  to  Twenty-fourth  Month  after  Birth. 
Decalcification  Begins,  Seventh  to  Eighth  Year. 

Shedding  Process  Takes  Place,  Eleventh  to  Twelfth  Year. 
Average  Length  of  Crown,  .21. 

Average  Length  of  Root,  .44. 

Average  Length  over  All,  .65. 

The  anatomy  of  this  tooth  being  almost  identical  to  that  of  the 
lower  first  permanent  molar,  it  will  be  unnecessary  to  enter  into  a  descrip- 
tion in  detail.  The  lobes,  and  consequently  the 
grooves,  are  the  same  in  position,  name,  and 
number,  and  a  similar  developmental  process  is 
recorded.  The  tooth  is  not  characterized  by  a 
prominent  cervical  ridge,  such  as  is  found  upon 
the  lower  first  molar,  the  crown  passing  very 
gradually  into  the  root-base  with  a  neck  moder- 
ately constricted. 

THE  PULP-CHAMBERS  AND  CANALS  OF 
THE  DECIDUOUS  TEETH. 

tig.     178. — Lower     Second 

Molar,  Buccal  Surface.  ^  few  genera}  remarks  in  reference  to  these 

cavities,  in  connection  with  the  information  to  be  derived  from  the 
accompanying  chart  and  its  annexed  description,  will  sufficiently  instruct 
the  reader,  without  the  necessity  of  treating  each  tooth  individually. 
The  pulp-chambers  and  canals,  like  those  of  the  permanent  organs; 
assume  the  form  of  the  external  contour  of  the  tooth,  the  crown  of  the 
tooth  being  provided  with  a  central  cavity,  the  pulp-chamber,  partaking 
of  outlines  closely  resembling  those  of  the  crown,  while  the  root  is 
traversed  by  the  pulp-canal,  likewise  conforming  to  the  shape  of  the 
root.  One  very  important  distinction  between  the  pulp-chambers  and 
canals  of  the  deciduous  teeth  and  those  of  the  permanent  organs  is  that 
the  former  are  proportionately  larger.  It  must  also  be  noted  that  the 
apical  foramina  in  these  teeth  are  so  transitory  in  their  nature  that  there 
remains  but  a  very  brief  period  during  which  the  canals  may  be  said 
to  be  fully  formed.  It  will  be  recalled  that  in  a  very  short  time  after 
the  roots  have  become  completely  calcified,  decalcification  begins,  and 


THE    DECIDUOUS    TEETH   IN    DETAIL 


23I 


this  process  of  degeneracy,  beginning  at  the  apical  extremities  of  the 
roots,  very  early  destroy  the  foramina,  which  have  in  a  measure  served  as 
a  protection  to  the  surrounding  parts  during  operations  upon  the  canals. 
With  the  canals  proportionately  larger  than  those  which  occupy  the 
roots  of  the  permanent  teeth,  the  foramina  during  their  very  limited 
existence  are  also  much  larger,  and  much  more  readily  penetrated. 
With  these  ever-changing  conditions  in  the  pulp-canals  of  the  deciduous 
teeth,  it  is  of  importance  that  a  definite  knowledge  of  what  takes  place 
should  be  acquired,  and  it  is  for  this  purpose  that  the  accompanying 
chart  has  been  prepared  (Fig.  179). 


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PART  II.-HISTOLOGY  AND  HISTOGENESIS. 


CHAPTER  I. 


General  Cytology;  General  Embryology;  and  Histogenesis. 

The  foregoing  pages  have  been  devoted  to  the  description  of  the  face 
and  oral  cavity,  so  far  as  they  can  be  studied  with  the  naked  eye,  either 
by  close  inspection,  superfically  as  it  were,  or  after  their  component  parts 
have  been  made  manifest  by  dissection.  That  part  constitutes  what  is 
commonly  known  as  gross  or  macroscopic  anatomy. 

Similar  to  all  other  regions  of  the  body,  however,  the  functions  of 
the  individual  parts  composing  the  face  and  the  oral  cavity,  in  health, 
and  the  pathological  processes  which  take  place  in  them  when  they  are 
diseased,  are  dependent  on  the  normal  or  pathological  condition  of  the 
minute  structures  or  tissues,  of  which  these  parts  are  composed.  These 
minute  structures  cannot  be  seen  with  the  naked  eye,  they  must  be  exam- 
ined with  the  aid  of  a  microscope,  and  this  part  of  study  is  therefore 
known  as  microscopical  anatomy  or  histology — the  science  of  tissues. 

The  minute  structures  of  the  face  and  oral  cavity,  generally  speaking, 
in  no  way  differ  from  the  structures  of  other  parts  of  the  body,  but  here, 
more  than  in  any  other  region  of  the  body,  the  anatomy  as  well  as  the 
histology  of  the  parts  cannot  be  well  understood,  unless  their  gradual  for- 
mation or  development — embryology — is  studied.  To  enable  the  student 
of  dentistry  to  understand  more  intelligently  the  details  of  the  parts  in 
which  he  is  most  interested,  it  is  therefore  essential  to  have  a  more  or  less 
thorough  knowledge  of  the  broad  principles  underlying,  not  only  the 
mature  structural  arrangements  of  the  human  body,  but  also  the  develop- 
ment of  the  organs — organogenesis — as  well  as  the  development  of  the 
tissues  of  which  these  organs  are  composed — histogenesis. 

In  submitting  parts  of  different  organs  of  the  body  to  a  more  close 
examination  with  the  microscope,  we  find  revealed  a  great  variety  of 
more  or  less  complex  textural  arrangements,  each  characteristic  of  the 
part  examined,  and  adapted  to  the  function  which  the  given  part  has  to 
fulfill.     Some  tissues  we  find  consisting  of  either  very  thin  or  more  or 

233 


234  HISTOLOGY 

less  coarse  fibers  running  parallel  to  each  other  in  a  compact  fashion  and 
forming  firm  or  elastic  bands;  others  we  find  showing  the  same  kind 
of  fibers  loosely  arranged  and  interlacing  with  each  other,  thus  forming 
more  or  less  typical  networks.  In  some  instances  we  find  similarly 
formed  elements,  so-called  cells,  joined  together  in  a  mosaic-like  fashion 
and  forming  either  coverings  of  surfaces  or  various  tubes  or  sacs.  The 
majority  of  organs  consist  of  fibers  and  cells,  modified  and  combined 
together  in  either  a  simple  or  more  or  less  complex  manner.  We  also 
observe  that  the  substances  filling  the  spaces  between  the  fibers  and 
cells,  the  so-called  ground  substances,  vary  in  their  consistency;  it  may  be 
a  fluid  or  a  semifluid,  it  may  be  of  a  more  or  less  firm  nature,  or  it  may 
be  very  hard.  Notwithstanding  this  great  variety,  however,  all  the 
tissues  of  which  the  animal  body  is  composed  can  be  grouped  into  five 
distinct  classes,  each  one  of  which  has  its  very  well  defined  character- 
istics of  structure,  which  makes  it  specially  adapted  to  the  various  func- 
tions which  it  has  to  fulfill  in  the  animal  economy.  These  classes  com- 
prise the  elementary  tissues,  which  by  their  various  modifications  and 
combinations  form  all  the  organs  of  the  body.  They  are  the  following: 
i.  Epithelial  tissue.  2.  Connective  tissue.  3'.  Muscular  tissue.  4.  Nerv- 
ous tissue.     5.  Blood  and  lymph. 

Before  we  attempt  to  describe  the  individual  elementary  tissues, 
it  is  necessary  to  become  familiar  with  some  features  which  are  common 
to  all  of  them.  This  can  be  best  accomplished  by  giving  a  brief  history 
of  the  beginning  and  gradual  development  of  our  knowledge  on  the  subject. 

The  principal  features  in  the  minute  structure  of  animals  are  the 
same  as  of  plants,  and  it  may  be  justly  stated  that  animal  histology 
took  its  origin  from  the  histology  of  plants.  In  1667  Robert  Hooke 
published  the  results  of  his  microscopical  investigations  of  a  piece  of  cork. 
He  found  that  it  consisted  of  a  number  of  small  boxes  or  cells,  and  thus 
the  name  cell  was  introduced  into  histologic  nomenclature.  In  1838 
the  botanist  Schleiden  published  the  results  of  his  elaborate  studies  of 
plant  structures,  and  established  the  fact  that,  no  matter  how  widely 
individual  plants  and  parts  of  plants  differ  from  one  another  in  their 
general  appearance,  in  their  minute  structure  they  reveal  themselves  as 
constituting  aggregations  of  cells,  or  modifications  of  such.  He  however 
laid  particular  stress  upon  the  fact  that  each  cell  contains  a  substance, 
semifluid  in  nature,  which  constitutes  the  essential,  life-carrying  part 
of  the  cell,  which  by  later  investigators  was  named  protoplasm.  Further- 
more, he  found  that  within  the  protoplasm  there  can  always  be  distin- 
guished a  kernel-like  body,  which  was  named  nucleus. 


GENERAL   CYTOLOGY;    EMBRYOLOGY;  AND  HISTOGENESIS 


235 


Inspired  by  personal  contact  with  Schleiden,  the  anatomist  Schwann 
undertook  extensive  examinations  of  various  tissues  of  animal  bodies. 
In  1839  he  published  the  results  of  his  investigations  and  established  the 
fact,  that  no  matter  how  widely  various  animals  and  organs  of  animals 
differ  in  regard  to  their  general  appearance,  in  their  minute  structure 
they  always  present  aggregations  of  cells,  or  modifications  of  such. 

This  conformity  in  the  results  of  the  investigations  of  Schleiden  and 
Schwann  has  revealed  the  most  interesting  as  well  as  the  most  important 
fact,  that  just  as  all  chemical  compounds  have  as  their  ultimate  units 


Fig.  180. — Cells  from  bulb  of  a  fresh  onion  forming  a  membrane.     M.  cell  membrane;  N ,  nu- 
cleus seen  from  the  surface;  Nn,  nucleolus;  V,  vacuoles,  X240.     (After  Stirling.) 

the  atoms  or  the  molecules,  so  all  living  tissues  have  as  their  ultimate 
units  the  cells.  This  forms  the  foundation  for  all  considerations  of 
plant  and  animal  histology  up  to  the  present  time,  notwithstanding 
the  recent  advancement  in  our  knowledge  of  the  structure  and  life  of  the 
cell  itself. 

According  to  the  views  of  Schleiden,  Schwann  and  their  contempo- 
raries, a  cell  consists  of:  (1)  a  cell-membrane;  {?.)  a  substance  contained 
within — a  protoplasm;  and  (3)  a  small  kernel-like  body — a  nucleus. 

Continuous  extensive  studies  of  the  structure  of  cells  in  various 
animal  tissues  have,  however,  very  soon  revealed  the  fact  that  while  in 
all  cells  there  can  always  be  demonstrated  a  protoplasm  and  a  nucleus, 
the  presence  of  a  cell-membrane  is  an  exceedingly  rare  occurrence,  and 
that  this  part  therefore  cannot  be  considered  as  an  essential  one  in  the 
structure  of  a  cell.     It  was  also  found  that  there  may  be  a  number  of 


236  HISTOLOGY 

other  structural  elements  present  in  one  or  the  other  kind  of  cells,  but  by 
their  absence  the  individuality  of  the  cell  is  by  no  means  lost.  The 
establishment  of  these  facts  naturally  led  to  a  change  in  the  conception  of 
the  cell-structure,  and  in  1861  a  distinguished  investigator,  Max  Schullze, 
narrowed  down  the  definition  of  a  cell  to  a  mass  of  protoplasm  containing 
a  nucleusu&nd  this  definition  is  generally  accepted  as  the  most  appropriate 
one  up  to  the  present  time. 

Another  very  important  point  pertaining  to  the  conception  of  the 
cell  has  attracted  the  attention  of  all  well-known  investigators  for  a  long 
time,  namely,  the  origin  of  cells  in  general,  and  t  e  formation  of  new  cells 
in  the  different  tissues  for  substituting  old  ones  when  they  are  worn  out 
or  lost  through  injury  or  disease.  It  was  at  one  time  believed  that  cells 
may  originate  spontaneously  by  transformation  from  organic  or  inorganic 
chemical  compounds.  By  persistent  investigations  it  was  however 
gradually  determined  that  the  so-called  spontaneous  generation  is  much 
less  frequent  than  had  at  first  been  supposed,  and  at  present  the  belief 
in  the  formation  of  cells  in  that  way  is  scarcely  held  by  anybody.  It  is 
definitely  established  that  all  varieties  of  animal  cells,  no  matter  to  which 
class  of  tissues  they  may  belong,  always  originate  from  preexisting  cells 
of  the  same  kind.  Furthermore,  it  was  ascertained  that  while  the 
different  tissues  of  the  animal  body  can  be  easily  distinguished  from  one 
another  in  their  mature  state,  there  are  great  similarities  between  them 
in  the  early  periods  of  the  development  of  the  animal.  These  similari- 
ties become  the  more  pronounced  the  further  back  we  trace  them  to 
their  starting-point,  and  find  that  all  tissues  collectively  take  their  origin 
from  the  same  source,  namely,  from  the  ovum  or  egg,  which,  according 
to  our  knowledge  at  present  time,  is  also  nothing  else  but  a  single  cell. 

The  polymorphism  of  cells,  or  the  great  variety  of  cell-forms,  which 
was  observed  in  the  animal  body,  has  naturally  suggested  the  assumption 
that  the  structure  of  cells  with  their  essential  and  accessory  parts  is  by  no 
means  a  simple  one,  and  further  investigations  have  proved  this  to 
be  correct.  The  study  of  cells  and  tissues  has  been  greatly  facilitated 
by  the  discovery  of  various  substances,  animal  as  well  as  vegetable,  and 
particularly  chemical  compounds,  which,  when  brought  in  contact  with 
the  tissues,  show  a  distinct  chemical  affinity  to  the  different  parts  of  it 
and  possess  the  ability  of  staining  them  in  a  distinct  characteristic  manner. 
Some  of  these  substances  stain  the  protoplasm  of  the  cells  and  are 
therefore  called  protoplasmic  stains,  others  stain  the  nucleus  and  are  there- 
fore called  nuclear  stains.  By  application  of  these  staining  materials  it 
was  made  possible  to  reveal  the  details  in  the  structure  of  the  various 


GENERAL    CYTOLOGY;    EMBRYOLOGY;    AND    HISTOGENESIS. 


237 


cells,  and  in  the  following  we  will  give  a  description  of  the  minute  struc- 
ture of  the  cells,  as  it  is  generally  accepted  at  present  time,  and  may  be 
observed  in  one  or  the  other  kind  of  physiologically  differentiated  cells 
during  the  various  stages  in  their  life-history. 

According  to  the  statement  made  above,  the  essential  parts  of  a 
cell  are  the  protoplasm  and  the  nucleus.  The  term  protoplasm  is  how- 
ever used  in  modern  histology  to  designate  the  substance  of  the  whole 
cell,  while  the  substance  of  the  cell-body  surrounding  the  nucleus  is 
termed  cytoplasm;  the  substance  of  the  nucleus  is  sometimes  termed 
nucleoplasm,  but  most  frequently  simply  nucleus.  We  will  also  use  these 
terms  in  our  description. 


Nuclear  membrane 


Achromatic 
substances 
of  the  nu- 
cleus 


Linin 


{        Nuclear  sap 


Cuticular  stratum 1 


Filar-mass 


Interfilar-mass -> 


'-Inclusions 


Netknots 


V Nucleolus 


Microsomes 


Chromatic 
substances 
of   the  nu- 
cleus 


•  Cell  membrane  (pellicula) 


Fig.  181. — Scheme  of'a  Cell.     Microsomes  and  filar-mass  only  partly  sketched.     (Stohr.) 


Cytoplasm. — The  structure  of  the  cytoplasm  has  been  described 
differently  by  different  investigators,  and  several  theories  have  been 
advanced  in  that  respect.  Some  have  described  it  to  be  of  an  homo- 
geneous nature;  others  found  it  to  be  reticular;  again,  it  was  claimed  to 
be  granular;  by  some  it  is  supposed  to  be  foam-like.  Thanks  to  experi- 
mental research-work  carried  out  within  recent  years,  it  is  positively 
known,  at  present,  that  the  most  important  of  all  vital  manifestations  of 
the  cells — metabolism — takes  place  in  the  cytoplasm,  and  this  being  the 
case  the  various  stages  of  the  process  naturally  manifest  themselves  in  the 
structure  of  the  cytoplasm.  In  this,  and  in  the  diversity  of  the  methods 
used  in  preparing  tissues  for  examination,  must  be  sought  the  cause  for 


238  HISTOLOGY 

the  difference  in  the  results  obtained.  There  is  a  general  agreement 
among  histologists,  however,  that  even  when  fresh,  unstained  tissues  are 
examined  there  can  be  recognized  in  the  cytoplasm  two  different  parts, 
one  forming  a  reticulum  or  network  and  called  spongioplasm;  the  other, 
a  substance  more  homogeneous  in  character,  which  fills  out  the  spaces 
of  the  network  and  is  called  hyaloplasm.  The  reticulum  of  the  spongio- 
plasm may  at  times  show  a  bead-like  structure,  thus  presenting  a  granular 
appearance.  The  hyaloplasm  may  also  be  not  entirely  homogeneous, 
but  contain  very  fine  granules  which  are  known  as  microsomes.  There 
may  be  also  found  in  the  cytoplasm  various  foreign  inclosures,  such  as 
particles  of  pigment,  droplets  of  oil,  some  apparently  empty  spaces  known 
as  vacuoles,  etc.  All  such  inclosures  are  collectively  known  as  meta-, 
para-  or  dentoplasm.  There  is  also  very  often  seen  a  condensation  of  the 
peripheral  part  of  the  cytoplasm,  constituting  what  is  called  an  ecto-  or 
exo plasm,  which  without  any  sharp  lines  of  demarcation  passes  into  the 
more  central  part,  called  endoplasm.  A  cell-wall  or  -membrane  must  be 
considered  as  a  highly  specialized  part  of  the  exoplasm. 

Nucleus. — In  the  great  majority  of  instances,  the  nucleus  is  a  spheri- 
cal or  oval  body,  situated  usually  either  in  the  center  of  the  cell  or  nearer 
to  one  pole  of  it  than  to  the  other,  and  varies  in  size  generally  in  propor- 
tion with  the  size  of  the  cell.  In  general  its  structure  may  be  said  to  be 
somewhat  similar  to  that  of  the  cytoplasm,  as  it  also  consists  of  a  reticu- 
lum or  network,  and  a  substance,  more  fluid  in  nature,  filling  the  meshes 
of  it.  The  network  of  the  nucleus  is  however  more  complicated  in  its 
structure.  By  means  of  staining  the  specimens  with  so-called  nuclear 
stains  it  has  been  revealed  that  the  nuclear  network  consists  of  two 
different  substances,  first,  of  an  exceedingly  delicate,  non-stainable  reticu- 
lum, apparently  very  similar  to  the  one  of  the  cytoplasm,  and  called 
linin;  secondly,  of  a  substance  which  has  the  form  of  threads,  or  of  a 
network,  or  of  a  mass  of  granules,  and  has  the  power  of  absorbing  very 
actively  certain  dyes.  This  latter  substance  has  therefore  received  the 
name  chromatin  substance  in  contradistinction  to  all  other  substances 
of  the  nucleus,  which  are  not  affected  by  the  dyes,  and  therefore  known 
as  oxhro  matin  substance. 

The  nucleus  is  separated  from  the  cytoplasm  by  a  distinct  line  of 
demarcation,  which  is  known  as  nuclear  membrane.  This  may  however 
disappear  at  times,  thus  making  possible  an  interchange  of  the  substance 
of  the  nucleus  with  that  of  the  cytoplasm.  It  generally  takes  place 
during  the  process  of  the  so-called  cell-division,  of  which  we  will  speak 
later. 


VITAL   MANIFESTATIONS    OF   CELLS  239 

Nucleolus. — This  is  a  small  body  found  in  the  center  of  the  nucleus 
in  the  great  majority  of  cells.  In  some  instances,  as  for  example  in  nerve 
cells,  it  becomes  very  conspicuous.  The  significance  of  it  in  the  cell- 
life  is  however  not  as  yet  well  established. 

Centrosome. — This  is  a  minute  body  observed  in  cells  in  close  relation 
to  the  nucleus  and  generally  situated  just  outside  of  the  nuclear  membrane. 
It  consists  of  a  more  homogeneous  substance  and  contains  in  its  center  a 
small  dot,  the  centriole.  The  centrosome  can  not  always  be  seen,  but  it 
becomes  very  conspicuous  during  a  certain  period  of  activity  of  the  cell, 
namely,  during  cell-multiplication,  when  it  plays  a  very  significant  role 

Other  constituents  of  cells  have  been  mentioned  already. 

VITAL  MANIFESTATIONS  OF  CELLS. 

In  animals  consisting  of  but  one  cell,  the  so-called  unicellular  organ- 
ism, all  life  functions  are  naturally  exerted  by  this  one  cell.  The  results 
of  microscopic  and  experimental  investigations  of  the  last  half  century 
have  however  led  to  the  conviction  that  the  life  activities  of  all  the  familiar 
higher  animals,  in  health  as  well  as  in  disease,  are  dependent  on  the 
activities  of  the  cells  of  which  they  consist.  It  is  for  that  reason  that, 
while  the  study  of  the  details  of  this  subject  belong  to  the  domain  of 
physiology,  some  of  the  more  important  points  of  it  are  also  generally 
considered  in  connection  with  the  description  of  cell-structure.  The 
most  conspicuous  vital  characteristics  of  cells  are  the  following: 

Metabolism. — Of  all  the  vital  activities  of  cells,  this  is  the  most  essen- 
tial one.  It  represents  the  sum  total  of  all  those  physico-chemical  proc- 
esses taking  place  in  cells  which  have  as  their  ultimate  result  the  main- 
tainance  of  the  life  and  individuality  of  the  cells.  It  therefore  embraces 
the  process  of  taking  into  the  body  material  suitable  for  its  nutrition; 
the  process  of  converting  some  of  it  into  a  part  of  its  own  substance  to 
replace  that  which  became  worn  out;  transformation  of  another  part 
into  various  kinds  of  energy;  finally,  the  elimination  of  subtsances,  not 
suitable  for  further  use,  as  waste-products. 

Growth. — Newly-formed  cells  are  generally  not  of  the  same  size  as 
mature  cells,  therefore  a  part  of  the  substances  assimilated  by  young  cells 
is  used  up  by  them  for  the  increase  of  their  size.  This  goes  hand  in  hand 
with  the  shaping  of  the  cell  until  it  becomes  identical  with  the  one  from 
which  it  originated. 

Irritability. — This  is  a  faculty  of  living  cells  to  respond  to  various 
influences:  mechanical,  chemical,  thermical,  etc.     It  generally  results  in 


240  HISTOLOGY 

those  peculiar  manifestations  of  the  cells  to  which  they  are  specially 
adapted,  for  example,  secretion,  motion,  etc. 

Conductivity. — This  is  the  ability  of  cells  to  convey  and  transmit 
impulses  from  one  part  of  the  cell  to  another  and  from  one  cell  to  another. 

Motion. — This  is  the  property  of  cells  to  change  their  relative  posi- 
tion to  the  surrounding  media.     It  occurs  in  three  different  forms: 

(1)  Amoeboid  Motion. — Similarly  to  the  amoeba,  cells  may  send  out  thin 
or  thick  projections  from  their  body,  called  pseudopodia,  and  if  such  pro- 
jections become  attached  to  some  foreign  object  and  the  rest  of  the  body 
is  drawn  after  it,  there  results  a  creeping  motion.  Cells  endowed  with 
such  motion  are  known  as  wandering  cells.  In  some  instances  such  cells 
may,  by  means  of  their  processes,  surround  foreign  bodies  and  either 
incorporate  them  by  assimilation  into  their  own  body-substance,  or 
deposit  them  by  expulsion  into  various  other  localities.  Such  wander- 
ing cells  are  known  as  phagocytes. 

(2)  Ciliary  Motion. — Some  cells,  usually  those  columnar  in  shape, 
have  one  of  their  free  surfaces  beset  with  a  number  of  delicate,  transparent, 
hair-like  processes  called  cilia,  which  are  constantly  lashing  forward 
and  backward  (reminding  of  the  blades  of  grass  in  a  field,  when  acted 
upon  by  a  strong  wind)  producing  a  strong  current,  thus  sweeping  along 
various  small  substances. 

(3)  Contractile  Motion. — This  is  generally  observed  in  fiber-shaped 
cells,  such  as  are  found  in  muscles.  It  consists  of  a  shortening  of  the  fibers 
in  the  direction  of  its  long  axis.  The  result  is  an  approximation  of  the  two 
ends  of  the  fiber  and  consequently  an  approximation  of  the  parts  to 
which  the  two  ends  are  attached. 

Reproduction. — While  all  the  just  mentioned  cell-properties  either 
serve  to  maintain  their  own  individuality,  or  are  the  manifestations  of 
their  function  in  the  animal  economy,  reproduction  is  the  property  of 
cells  to  form  new  cells  similar  to  their  own,  a  kind  of  rejuvenation,  as  it 
were.  The  object  of  this  is  either  to  substitute  new  cells  for  the  ones 
which  have  become  worn  out  and  cast  off,  or,  by  adding  new  ones  to  their 
own  ranks,  increase  their  functional  ability. 

The  studies  of  the  structural  changes  which  take  place  in  cells 
during  cell-reproduction,  have  been  pursued  with  great  zeal  and  enthu- 
siasm during  the  last  four  or  five  decades,  because  it  was  through  these 
studies  that  the  true  and  complex  structure  of  cells  was  revealed.  We 
will  therefore  consider  this  phenomenon  in  its  details  at  this  point. 

The  cell-structure,  as  described  on  page  238,  is  considered  as  the 
one  which  presents  the  cell  in  its  so-called  resting  state,  and  the  changes 


VITAL   MANIFESTATIONS    OF   CELLS 


241 


which  have  been  observed  in  it  during  the  process  of  reproduction  are 
the  following: 

In  the  first  place,  the  centrosome,  whether  it  has  previously  been  visible 
or  not,  becomes  very  distinctly  defined,  and  divides  in  two,  which  grad- 
ually separate  from  one  another,  and  ultimately  pass  to  opposite  poles 


/--■chr.  ',  ^ 


Y.-sp. 


Fig.  182. — Diagram  of  indirect  or  mitotic  cell- division;  A,  cell  with  nucleus  in  resting  stage; 
B,  cell  with  nucleus  in  skein  stage  and  with  centrosomes  separating;  C,  formation  of  chromo- 
somes; D,  longitudinal  splitting  of  the  chromosomes;  E,  separation  of  the  chromosomes; 
F,  diaster  stage;  G,  H,  formation  of  the  two  daughter  cells;  c,  centrosome;  cl,  chromatin  thread; 
chr.,  chromosomes;  nu,  nucleus;  n,  nucleolus;  sp.,  nuclear  spindle;  w,  cell  wall.  (After 
Galloway.) 

of  the  cell;  they  remain  connected  with  each  other  however  by  means 
of  finely  drawn-out  non-stainable  fibers,  known  as  the  achromatic  spindles. 
In  the  meantime  the  nuclear  membrane  disappears  and  the  chroma- 
tin substance  of  the  nucleus,  whatever  be  its  form  during  the  resting 
stage,  assumes  the  form  of  a  continuous,  densely  coiled-up  thread,  which 
16 


242  HISTOLOGY 

is  known  as  the  close  skein.  This  coil  gradually  becomes  in  a  measure 
entangled,  and  is  then  known  as  the  loose  skein. 

The  next  step  is  one  of  the  most  significant  ones  in  the  whole  process. 
The  whole  chromatin  thread  becomes  broken  up  into  a  number  of  seg- 
ments known  as  chromosomes,  and  it  is  a  very  important  fact  that,  while 
the  number  of  chromosomes  varies  in  cells  of  animals  of  different  species, 
there  is  always  an  equal  number  of  them  in  cells  of  animals  of  the  same 
species.  The  chromosomes  become  arranged  in  the  equatorial  plane 
of  the  spindle  in  a  somewhat  star-like  fashion,  and  this  is  then  spoken  of 
as  the  monaster  stage.  Next,  a  splitting  of  each  individual  chromosome 
in  its  longitudinal  direction  takes  place,  and  in  this  way  the  number  of 
chromosomes  becomes  doubled,  each  one  consisting  now  of  a  pair,  or 
presenting  a  twin-chromosome,  as  it  were.  The  two  chromosomes  of 
each  pair  gradually  separate  from  one  another  and  one  of  each  pair  ulti- 
mately passes  to  opposite  poles  of  the  spindle,  near  the  centrosome.  Here 
they  also  assume  a  star-like  arrangement,  and  we  have  there  what  is 
known  as  the  Master -stage.  The  chromosomes  are  generally  regarded  at 
the  present  time  as  the  bearers  of  heredity. 

The  changes,  which  take  place  in  each  one  of  the  stars,  from  now  on 
present  simply  the  reverse  of  those  described  above  for  the  formation  of 
the  monaster,  i.e.,  the  individual  chromosomes  become  united  with  one 
another,  thus  forming  two  skeins.  At  first  the  skeins  are  loose,  then  a 
condensation  takes  place  and  two  close  skeins  are  formed.  Finally, 
around  each  one  of  them  a  membrane  is  formed,  and  there  appear  two 
nuclei  similar  to  the  one  from  which  they  originated. 

While  this  final  shaping  of  the  nuclei  is  going  on,  the  cytoplasm  of 
the  cell  becomes  constricted  half-way  between  the  two  nuclei,  and  when 
this  is  completed  a  division  has  thus  taken  place,  and  we  have  two  cells 
instead  of  the  original  one. 

The  various  investigators  have  laid  particular  stress  on  one  or  the 
other  stage  during  the  process  of  cell-multiplication.  Accordingly,  various 
names  have  been  given  to  the  same  process,  they  all  have  their  justifica- 
tion and  are  in  use  as  synonyms.  Taking  into  consideration  the  ultimate 
result  of  the  process,  it  is  called  cell- division.  In  view  of  the  fact  that 
the  changes  are  most  conspicuously  going  on  in  the  nucleus,  it  has  been 
spoken  of  as  karyokynesis.  Because  the  chromatin  substance  of  the 
nucleus  assumes  the  form  of  a  thread,  the  process  is  called  mitosis.  Finally, 
as  this  form  of  cell-division  is  accomplished  in  a  rather  complicated 
round-about  way,  it  is  also  known  as  indirect  cell-division. 

While  the  process  of  cell-division  just  described  is  the  most  frequently 


ORGANS  AND   TISSUES 


243 


met  with,  there  can  be  observed  a  more  simple  one,  which  leads  to  the 
same  end.  Without  any  preliminary  rearrangement  in  its  structure,  the 
nucleus  of  the  cell  becomes  elongated,  then  it  assumes  a  dumb-bell  shape, 
finally  the  connecting  neck  becomes  broken  across  and  thus  two  so-called 
daughter  nuclei  are  formed  which  gradually  separate  from  one  another; 
in  the  meantime  a  constriction  of  the  cytoplasm  takes  place,  and  as  a 
result  there  appear  two  cells  instead  of  one.  For  the  reason  that  in 
this  form  of  cell-division  the  nuclear  substance  does  not  assume  any 
thread-like  arrangement,  and  the  end  is  accomplished  in  a  rather  simple 


Fig.  183. — Direct  cell  division  (Amceba). — A,  active  specimen  with  pseudopodia;l  becom- 
ing spherical  preliminary  to  division;  C,  beginning  of  elongation  and  constriction;  D,  later 
stage;  E,  daughter  cells  forming  pseudopodia;  ec,  exoplasm;  en,  endoplasm;/,  food  particle; 
n,  nucleus;  ps,  pseudopodium;  v,  vacuole.     (After  Galloway.) 

manner,  this  process  is  called  amitosis  or  direct  cell-division.  This 
process  has  been  observed  in  the  white  corpuscles  of  the  blood,  occasion- 
ally in  the  liver,  and  not  infrequently  in  pathologic  conditions;  it  is  assumed 
that  this  mostly  occurs  in  cells  having  a  lowered  vitality,  there  is  however 
much  to  be  learned  yet  in  regard  to  this  question. 


ORGANS  AND  TISSUES. 


All  through  the  animal  kingdom,  with  the  exception  of  the  unicellular 
organisms,  we  observe,  among  the  various  elements  of  which  they  consist, 
the  manifestation  of  a  phenomenon  commonly  known  as  division  of 
labor.  We  find  groups  of  cells  united  together  in  various  fashions  to 
perform  a  certain  function,  and  such  aggregations  of  cells,  specialized  to 
fulfill  well-defined  duties  in  the  economy  of  the  organism,  are  called  organs. 
Only  among  the  lower  animals,  however,  do  we  find  that  the  organs  consist 


244  HISTOLOGY 

of  only  one  kind  of  cell.  Higher  in  the  scale  of  animal  organization,  we 
find  the  organs  exhibiting  more  and  more  complexity  in  their  make-up. 
They  consist  of  groups  of  various  kinds  of  cells,  and  the  individual  groups 
present  very  little  similarity  to  each  other,  sometimes  becoming  modified 
to  such  an  extent  that  it  is  very  difficult  to  recognize  the  cellular  nature 
of  them.  While  the  name  organ  conveys  the  idea  of  a  physiological  unit, 
the  texture  of  it  is  known  as  tissue.  If  the  tissue  presents  an  aggregation 
of  elements,  similar  in  character,  we  speak  of  simple  tissues;  and  where 
we  have  an  aggregation  of  elements  of  various  kinds,  we  speak  of  complex 
tissues. 

To  gain  a  proper  understanding  of  the  characteristic  appearance  of 
the  various  simple  tissues,  their  various  modifications,  combinations  and 
transformations  in  forming  that  multitude  of  structures,  which  is  ob- 
served in  a  mature  organism,  it  is  necessary  to  trace  them  to  their  first 
beginning,  or  histogenesis. 

We  have  stated  elsewhere  that  an  animal  body  takes  its  origin  from 
the  egg,  ovum,  which  is  nothing  else  but  a  single  cell.  While  ova  of 
various  types  of  animals  differ  in  size,  they  all  have  relatively  the  same 
structure  and  are  all  specialized  for  the  same  purpose — to  produce  a  new 
individual.  The  changes  which  an  ovum  undergoes  in  course  of  its 
development  into  a  new  individual  are  in  all  cases  principally  the  same, 
and  therefore  the  knowledge  gained  from  the  study  of  one  form  serves  us, 
in  a  general  way,  to  understand  all  others.  The  study  of  these  consecu- 
tive changes  constitutes  the  subject  of  General  Embryology  and 
Histogenesis. 

General  Embryology  and  Histogenesis. 

In  all  animals  with  sexual  mode  of  reproduction,  the  cell  from 
which  the  whole  organism  ultimately  develops  is  itself  a  product  of 
the  union  of  two  highly  differentiated  and  specialized  cells;  one,  supplied 
by  the  male  individual  and  formed  in  the  testicle — the  spermatozoon;  the 
other,  supplied  by  the  female  individual  and  formed  in  the  ovary — the 
ovum.  These  two  so-called  elements  of  reproduction  are  derived  by  trans- 
formation of  special  cells  in  the  body,  the  so-called  germinal  cells,  which 
are  set  aside,  as  it  were,  for  the  purpose  to  be  eliminated  from  the  body 
at  certain  periods  and  used  as  a  foundation  for  the  propagation  of  the 
species.  The  process  of  entrance  of  the  spermatozoon  into  the  ovum, 
and  the  ultimate  union  and  fusion  of  the  two  elements  is  known  as 
the  fertilization.     Before  this  fertilization  can  take  place,  however,  the 


GENERAL   EMBRYOLOGY  AND  HISTOGENESIS 


245 


ovum  has  to  undergo  a  preliminary  process  called  maturation,  which 
consists  in  the  elimination  of  half  of  the  quantity  of  its  nuclear  substance 
in  form  of  two  small  bodies,  expulsed  at  one  of  the  poles  of  the  ovum, 
and  therefore  called  polar  bodies.1  The  fertilized  ovum  is  that  cell  from 
which  a  new  being  gradually  develops;  and  while  the  mode  of  develop- 
ment varies  somewhat  in  its  details  in  different  species  of  animals,  the 
general  principles  of  the  process  always  remain  the  same.  In  the  follow- 
ing we  endeavor  to  give  a  short  general  account  of  the  essential  points 


-s*. 


**.- 


Fig.  184. — Four  stages  in  the  maturation  and  fertilization  of  the  ovum  (partly  diagram- 
matic). A,  formation  of  the  polar  bodies  and  entrance  of  the  spermatozoon;  B,  the  male  and 
female  pronuclei;  C,  nuclei  coming  together;  D,  pronuclei  uniting  to -form  segmentation 
nucleus;  e.  «.,  egg  nucleus;  p.  b.,  polar  bodies;  s,  spermatozoon;  s.  c,  sperm  centrosome; 
s.  n.,  sperm  nucleus;  5.  e.,  segmentation  nucleus  produced  by  the  union.     (After  Galloway.) 


in  the  development  of  the  ovum,  but  for  the  details  of  it  we  must  refer 
to  the  various  text-books  on  embryology. 

After  a  short  period  of  rest,  the  fertilized  ovum  begins  to  undergo 
the  so-called  process  cleavage  or  segmentation.  By  means  of  karyo- 
kinesis  it  divides  into  two  halves,  producing  two  cells.  Each  of  these  cells 
in  turn  again  divides,  giving  rise  to  four  cells,  and  this  is  succeeded 
by  another  division,  forming  eight  cells,  and  by  repeated  division  of  this 
kind  there  arises  a  solid  mass  of  smaller  cells  called  morula  or  mulberry 
mass,  from  its  resemblance  to  a  berry.  As  the  cells  increase  in  number, 
the  mass  also  increases  in  size  by  the  absorption  of  nutriment,  and  a 

1  By  expulsion  of  the  polar  bodies  the  number  of  chromosomes  in  the  ovum  is  reduced 
to  one-half  of  it;  the  fusion  with  the  spermatozoon,  which  is  also  supplied  onlv  with  half  the 
number  of  chromosomes,  the  normal  quantity  is  restored  and  the  fertilized  ovum  thus  contains 
heredity  bearing  substance  from  both  its  progenitors. 


246 


HISTOLOGY 


gradual  arrangement  of  the  cells  in  a  definite  fashion  takes  place,  varying 
however  with  the  character  of  the  ovum.     They  may  arrange  themselves 


Neph 


Fig.  185. — Diagrams  showing  the  development  of  the  germ  layers.  {After  van  Beneden 
andStohr.)  A,  Two-celled  stage;  B,  four-celled  stage;  C,  morula  stage;  D  to  I,  transverse 
section  of  various  stages  of  the  blastoderm. 

in  the  form  of  a  layer  of  cells  spherically  surrounding  the  so-called  seg- 
mentation cavity;  or  they  may  be  spherically  arranged  around  a  mass  of 
yolk;  or  they  may  form  a  disk-like  arrangement,  floating  as  it  were  on 


GENERAL   EMBRYOLOGY   AND    HISTOGENESIS  247 

a  spherical  mass  of  yolk.  In  anv  case  we  observe  the  cells  to  be  cylin- 
drical in  shape  and,  lying  side  by  side  in  regular  fashion,  form  a  some- 
what skin-like  arrangement.  This  is  therefore  called  the  blastoderm, 
which  means  a  germinal  skin,  and  the  individual  cells  are  named  blasto- 
meres.  The  continuous  multiplication  of  the  cells  of  the  blastoderm 
results  in  either  an  infolding  (invagination)  or  a  splitting  off  (delami- 
nation)  of  some  of  the  cells,  which  by  their  own  gradual  multiplication 
form  a  distinct  continuous  layer  of  cells  beneath  the  first  one.  It  is 
then  generally  spoken  of  as  a  two-layered  blastoderm,  of  which  the 
outer  layer  is  called  ectoderm  or  epiblast,  and  then  the  inner  layer  entoderm 
or  hypoblast.  These  two  layers  are  called  the  primary  germinal 
layers,  and  the  cells  of  which  they  consist  show  great  similarities  in 
many  respects.  Very  soon,  however,  some  of  the  cells,  on  the  surfaces 
of  the  two  primary  layers  which  face  each  other,  detach  themselves 
therefrom  by  the  process  of  splitting  off  or  delamination  and  lodge  in 
the  gradually  widening  space  between  the  layers.  As  the  number  of 
these  cells  increases  also  through  their  own  multiplication,  it  very  soon 
becomes  possible  to  distinctly  recognize  a  third  layer  of  cells,  which,  on 
account  of  its  being  situated  between  the  other  two,  has  received  the 
name  of  middle  layer  or  mesoderm  or  mesoblast.  With  the  formation 
of  the  mesoderm,  the  blastoderm  is  said  to  consist  of  three  germinal  layers, 
and  with  this  the  foundation  for  the  development  of  the  various  tissues 
and  organs  is  finally  established.  The  resemblance  of  the  cells  consti- 
tuting the  different  layers  gradually  becomes  more  and  more  lost,  and 
differentiation  then  begins  to  take  place.  Through  all  the  consecutive 
changes,  the  derivatives  of  the  ectoderm  and  entoderm  retain  the  charac- 
teristic tendency  to  be  arranged  in  groups  of  cells  lying  closely  side  by 
side  in  a  simple  or  stratified  fashion.  The  derivatives  of  the  mesoderm 
are,  on  the  contrary,  characterized  by  a  looser  arrangement  of  their  cells, 
some  of  which  remain  connected  with  one  another  by  means  of  proto- 
plasmic prolongations  or  processes,  while  others  retain  the  faculty  of 
wandering  away  and  intermingling  with  the  derivatives  of  the  other  two 
layers.  Each  of  the  three  layers  gives  rise  to  well-defined  structures 
adapted  to  distinct  physiological  functions,  but  none  of  them  can  in  this 
respect  be  substituted  by  another,  without  producing  abnormal  conditions 
in  the  organism. 


CHAPTER  II. 

Elementary    Tissues:  Epithelial    Tissue,    Connective    Tissue, 
Muscular  Tissue,  Nervous  Tissue,  Blood  and  Lymph. 

The  differentiation  of  the  three  layers  of  the  blastoderm  into  various 
tissues  goes  on  parallel  with  the  development  of  the  body  in  general,  and 
with  the  growing  complexity  of  organization  along  the  scale  of  the  animal 


-yn£  ^ 


Fig.  186. — A,  Diagram  of  a  longitudinal  section  through  the  body  of  a  Hydra;  it  presents 
the  structure  of  an  animal,  in  which  the  walls  consist  of  only  two  layers  of  cells  specialized  to 
perform  all  the  function.  B,  a  small  portion  of  the  wall  more  highly  magnified.  (After 
Galloway.) 

kingdom,  the  number  of  varieties  of  tissues  as  well  as  their  complexity 
also  increases.  On  page  234  we  have  already  indicated,  however,  that 
we  can  resolve  all  the  various  tissues  into  five  well-characterized  typical 
groups,  which  are  known  as  elementary  tissues.     These  are:  (1)  epithelial 

248 


EPITHELIAL    TISSUE  249 

tissue,  (2)  connective  tissue,  (3)  muscular  tissue,  (4)  nervous  tissue, 
(5)  blood  and  lymph. 

Among  the  lower  animals  there  are  some  which  resemble  to  a  con- 
siderable extent  the  early  embryonic  stages  of  higher  animals.  Their 
walls  consist  of  two  layers  of  cells,  which  resemble  one  another  very 
much,  and  are  transformations  respectively  from  the  ectoderm  and 
entoderm.  They  are  endowed  with  the  ability  to  perform  all  functions 
which  characterize  living  bodies  and  represent  that  typical  group  of  cells 
known  as  epithelial  tissue.  Accordingly,  in  the  higher  animals,  also,  the 
principal  functionating  parts  of  the  various  organs,  including  the  nervous 
system,  are  by  various  transformations  derived  from  the  ectoderm  or 
entoderm,  and  are  either  epithelial  tissue  proper  or  highly  specialized 
modifications  of  it. 

The  mesoderm  begins  to  develop  when  the  other  two  layers  are 
already  well  differentiated,  and,  accordingly,  the  tissues  which  take 
their  origin  from  that  layer  are  found  very  sparingly,  or  not  at  all,  in 
lower  animals,  but  become  more  and  more  conspicuous  as  we  ascend  in 
the  complexity  of  animal  organization.  It  gives  rise  mainly  to  tissues 
which,  while  not  constituting  the  principal  parts  in  the  texture  of  various 
organs,  are  nevertheless  of  very  gerat  importance  to  them  and  form  indis- 
pensable auxiliary  parts  of  them.  These  are  the  connective  tissues, 
muscular  tissue  and  blood  and  lymph.  Some  epithelial  formations  take 
their  origin  in  the  mesoderm  also,  but  their  number  is  very  limited. 

We  will  now  consider  the  individual  tissues  in  detail. 

I.  EPITHELIAL  TISSUE. 

Epithelial  tissue  is  found  as  coverings  of  all  surfaces  of  the  body, 
those  directly  exposed  to  the  air  as  well  as  those  which  form  variously 
shaped  cavities  and  communicate  with  the  air  indirectly  through  narrow 
or  wide  openings.  The  cells  composing  the  epithelial  tissue  are  known  as 
epithelial  cells.  These  are  definite  in  outline,  show  very  clearly  a  cyto- 
plasm and  a  nucleus,  and  lie  side  by  side  in  a  regular  fashion.  There  is 
just  enough  substance — so-called  intercellular  substance — between  the  indi 
vidual  cells  to  hold  the  cells  together.  Three  principal  forms  of  cells  are 
generally  met  with:  (1)  the  flattened  or  squamous,  (2)  the  cylindrical  or 
columnar,  and  (3)  the  many-sided  or  polyhedral.  If  a  single  layer  of  cells 
is  present,  it  is  known  as  simple  epithelium;  if  there  are  two  or  more  super- 
posed layers  of  cells  present,  it  is  spoken  of  as  stratified  epithelium.  In 
simple  squamous  epithelium  the  cells  are  flattened  or  scaly  and  the 


250  HISTOLOGY 

nuclei  are  round  and  also  flattened.  In  the  stratified  squamous  variety, 
which  is  the  one  most  frequently  met  with,  only  the  superficial  layers  are 
squamous,  while  the  deeper  ones  are  more  irregular,  and  may  gradually 
become  columnar. 

In  columnar  epithelium  there  is  quite  a  variety  in  the  outline  of  the 
cells.  In  simple  columnar  epithelium  the  cells  may  be  either  long — 
high  columnar — or  of  a  medium  size — cuboidal — -or  very  short — low  colum- 
nar. They  may  be  beset  on  their  free  surfaces  with  numerous  minute 
hair-like  processes,  which  are  constantly  vibrating  during  life  and  are 
known  then  as  ciliated  epithelium.  In  the  stratified  variety  also  the 
superficial  layer  only  may  be  typically  columnar,  while  the- others  may  be 
of  a  different  shape.  Among  a  continuous  superficial  layer  of  columnar 
epithelium,  there  may  be  found  scattered  here  and  there  cells  which  have 
somewhat  the  shape  of  a  conical  cup  and  contain  in  their  cytoplasm  mucus 
in  various  states  of  formation.  From  time  to  time  a  contraction  of  their 
cell-body  takes  place,  and  their  content,  which  is  of  a  mucous  character, 
is  poured  out  upon  the  surface.  Owing  to  their  shape,  these  cells  are 
called  goblet  cells. 

The  polyhedral  epithelium  may  be  found  in  various  localities  in  the 
body.  If  the  body  of  the  cells  contains  some  pigment  granules,  it  is 
known  as  pigmented  epithelium.  When  polyhedral  or  columnar  epithe- 
lium is  forming  the  constituting  part  of  the  so-called  glandular  organs,  it 
is  known  as  glandular  epithelium.  A  very  conspicuous  variety  of  epithe- 
lial cells  of  various  shapes  is  represented  in  the  so-called  neuro-epithelium, 
which  is  specialized  for  the  creation  and  perception  of  the  special  senses. 

Epithelial  tissue  never  contains  blood-vessels;  their  nutrition  takes 
place  by  the  absorption  of  nutritive  juices  through  the  clefts  between 
the  cells,  or  the  cement-substance.  It  is  obvious  that  when  a  stratified 
epithelium  consists  of  a  large  number  of  layers,  the  superficial  ones  may 
receive  very  little  nourishment  or  none  at  all,  which  accounts  for  the 
constant  exfoliation  of  cells  from  the  surface  skin  and  other  parts. 

II.  CONNECTIVE  TISSUE. 

Connective  tissue  is  the  most  widely  distributed  tissue  in  the  animal 
body.  It  holds  the  individual  parts  of  other  tissues  together;  it  connects 
the  various  tissues  with  one  another,  and  at  the  same  time  keeps  them 
separated  from  one  another;  it  gives  firmness  to  the  body  as  a  whole,  and 
a  support  for  the  various  organs  within  it;  it  forms  variously  constructed 
channels  for  the  distribution  of  the  nutritive  material  to  the  various  parts 


CONNECTIVE    TISSUE 


251 


of  the  body.  With  the  variety  of  functions  just  enumerated  there  is  a 
corresponding  variety  of  forms  in  which  connective  tissue  is  found  repre- 
sented in  the  body. 

We  have  stated  above  that  connective  tissue  takes  its  origin  from 
the  mesoderm.  With  the  advance  in  the  differentiation  of  the  ectoderm 
and  entoderm  the  derivatives  of  these  two  layers  gradually  become 
separated  from  one  another,  and  the  changes  of  the  mesoderm  follow 
closely  in  their  steps.  It  becomes  split  into  two  secondary  layers,  one  of 
which  attaches  itself  to  the  derivatives  of  the  ectoderm,  ready  to  serve 
them  with  the  above-enumerated  functions,  and  is  called  parietal  meso- 
derm, while  the  other  plays  the  same  role  in  regard  to  the  derivatives  of 


n.c.    ; 


—ect. 


md. 


Fig.  187. — Diagrams  of  transverse  sections  through  the  body  of  an  embryo  of  a  vertebrate. 
It  shows  the  relation  of  the  three  germinal  layers,  ect.,  Ectoderm;  ent.,  entoderm;  mes., 
mesoderm;  coe.,  coelom  or  body  cavity;  sk.,  beginning  of  skeleton;  «.,  beginning  of  spinal 
cord;  g.  lumen  of  the  gut.     (After  Gallcnvay.) 


the  entoderm,  and  is  called  visceral  mesoderm.  The  gap,  which  remains 
between  these  two  secondary  layers,  is  known  as  the  body-cavity  or 
coelom.  The  characteristic  feature  of  the  mesoderm  tissue  is  that  the 
cells  do  not  lie  side  by  side  and  form  continuous  layers,  but  are  separated 
from  one  another,  sometimes  quite  considerably,  and  the  spaces  between 
the  cells  are  occupied  by  a  substance  somewhat  gelatinous  in  consistency 
which  is  called  intercellular  substance  and  is  obviously  a  product  of  the 
cells  themselves.  The  cells  are  stellated  or  spindle-shaped,  and  their 
thinned-out  processes  unite  and  interlace  with  one  another,  forming  a 
network.  The  formation  of  the  various  forms  of  connective  tissue  from 
the  mesoderm  in  course  of  development  is  due  mainly  to  the  differentia- 


252  HISTOLOGY 

tion  and  various  chemical  changes  which  that  intercellular  substance 
or  matrix  undergoes.  In  view  of  the  fact  that  the  intercellular  substance 
constitutes  the  predominating  part  in  connective  tissue,  it  is  the  fea- 
ture upon  which  the  classification  of  this  tissue  is  based,  and  we  can  dis- 
tinguish five  well-defined  characteristic  groups:  (1)  mucous  or  embryonic 
connective  tissue,  (2)  ordinary  or  fibrous  connective  tissue,  (3)  cartilage, 
(4)  bone,  (5)  dentin. 

Mucous  or  Embryonic  Connective  Tissue. — This  tissue  closely 
resembles  in  its  structure  the  mesoderm  tissue  in  earliest  stages  of  the 
embryo,  as  described  above.  It  consists  of  a  semi-gelatinous,  mucoid 
matrix,  and  within  it  are  scattered  stellate  or  spindle-shaped  cells,  and 
here  and  there  thin  fibers.  The  latter  present  mainly  the  elongated  and 
anastomosing  processes  of  the  cells,  some  of  them  however  are  un- 
doubtedly products  of  the  matrix  itself,  by  a  process  that  reminds  some- 
what of  coagulation.  This  tissue  is  found  at  birth  in  the  umbilical  cord 
as  the  so-called  jelly  of  Wharton;  in  the  adult  human  body  it  is  found  in 
the  pulp  of  the  teeth  and  in  the  vitreous  humor  of  the  eye. 

Ordinary  or  Fibrous  Connective  Tissue. — This  tissue  is  present  in 
the  skin  and  mucous  membrane,  in  the  intermuscular  tissues,  in  tendons, 
in  fascia  and  aponeuroses,  and  in  the  tissues  connecting  various  organs. 
It  is  composed  of  a  meshwork  of  fine  fibers  of  two  kinds.  The  first,  which 
makes  up  the  greater  part  of  the  tissue,  is  formed  of  very  fine,  white, 
structureless  fibers  arranged  closely  in  bundles  and  bands  crossing 
and  intersecting  in  a'l  directions.  The  second  variety,  or  the  yellow, 
elastic  fiber,  has  a  much  sharper  and  darker  outline,  not  arranged  in 
bundles,  but  is  intimately  mingled  with  the  white  fibers  by  twisting 
around  and  among  its  filaments.  These  are  known  as  the  elementary 
connective-tissue  fibers.  The  size  of  the  connective-tissue  bundles 
depends  upon  the  number  of  elementary  fibers  present,  and  by  a  variation 
in  the  arrangement  of  the  bundles  variety  in  the  character  of  the  fibro- 
connective  tissue  is  produced  in  different  localities.  When  the  fibrous 
connective  tissue  is  formed  into  an  unbroken  mass,  as  in  mucous  mem- 
brane, the  minute  bundles  are  collected  into  smaller  or  larger  groups 
(the  trabecidce),  and  these  are  in  turn  associated  into  groups.  In  the  skin 
and  mucous  and  serous  membranes,  the  trabecular  of  the  connective- 
tissue  bundles  are  separated,  and,  by  crossing  and  recrossing  one  another, 
form  a  dense,  fan-like  structure.  In  other  tissues,  as  the  tendons  and 
fascia,  the  bundles  are  arranged  in  parallel  layers.  In  the  submucous 
tissues  the  connective-tissue  fibers  are  loosely  woven,  the  fibers  crossing 
and  intermingling,  with  the  intervening  spaces  unusually  large,  resulting 


CARTILAGE  253 

in  a  loose,  flabby  tissue.  Two  varieties  of  fibrous  connective  tissue  are 
distinguished — namely,  (a)  Compact  or  fibrillar  connective  tissue,  form- 
ing bands  of  either  white  fibers,  or  yellow  elastic  fibers,  or  mixed  fibers; 
and  (b)  loose  or  areolar  connective,  forming  a  network  or  reticulum. 
Modifications  of  the  loose  variety  are  found  represented  in  adipose  tissue 
and  lymphoid  tissue. 

The  fibrous  connective-tissue  cells  are  few  in  number,  of  several 
varieties,  and  variously  shaped,  being  flattened,  stellate,  or  apparently 
distorted  by  pressure  from  surrounding  cells  or  fibrous  bundles.  In 
the  mucous  membrane  the  cells  are  oblong  and  somewhat  flattened, 
having  many  branches  which  reach  out  and,  uniting  with  like  processes 
from  neighboring  cells,  form  a  network.  Other  connective-tissue  cells 
are  comparatively  larger,  oval  or  rounded  in  form,  granular  in  appearance, 
rich  in  protoplasm,  and  are  known  as  plasma-cells.  The  body  of  connect- 
ive-tissue cells,  besides  containing  a  nucleus,  frequently  contains  pigment- 
granules;  these  are  known  as  pigment-cells.  These  are  seldom  found  in 
mucous  or  serous  membranes,  being  principally  confined  to  the  integu- 
ment. Fat-globules  may  also  be  found  in  fibrous  connective  tissue,  and 
when  of  considerable  size  unite  and  form  a  rounded  cell,  called  a  fat-cell. 
Numerous  fat-cells  uniting,  and  well  supplied  with  blood-vessels  and 
nerves,  form  adipose  tissue,  or  fat.  Fat-cells  are  frequently  found  in 
areolar  tissue  as  well.  When  fibrous  connective  tissue  is  immediately 
contiguous  to  epithelium,  it  becomes  somewhat  modified  and  a  new 
membrane  is  formed,  called  the  basement  membrane,  or  membrana  propria. 
This  membrane  is  a  thin,  transparent,  structureless  layer,  and,  when  in 
connection  with  those  mucous  membranes  provided  with  a  layer  of  vas- 
cular fibrocellular  tissue,  may  appear  as  the  formative  substance  out  of 
which  successive  layers  of  epithelial  cells  are  generated.  In  the  ducts 
and  glands — for  example,  the  salivary  glands — the  basement  membrane 
forms  the  proper  walls  of  the  tubes,  and  the  cells  here  generated,  and 
corresponding  to  the  epithelial  cells  of  the  coarser  mucous  membranes, 
are  known  as  gland-cells,  rather  than  epithelial  cells.  This,  however, 
is  a  distinction  without  a  perceptible  difference,  the  location  and  function 
as  secreting  cells  being  alike  in  each. 

Cartilage. — Cartilage  is  a  semi-opaque,  non-vascular  tissue,  white 
in  color,  and  composed  of  a  matrix  containing  nucleated  cells.  The 
matrix  is  somewhat  elastic  and  rather  dense.  The  cells  are  simple  in 
form,  being  spheric  or  slightly  inclined  to  angularity.  The  variation  in 
the  character  of  cartilage  is  due  rather  to  the  difference  in  the  character 
of  the  matrix  than  to  the  cellular  structure,  the  principal  variation  in  the 


254  HISTOLOGY 

cells  being  in  their  size.  The  cells  lie  in  the  spaces  or  lacunae  of  the 
matrix,  which  they  completely  fill.  Investing  the  free  surface  of  most 
cartilaginous  tissue  (articular  cartilage  excepted)  is  a  thin  but  tough 
and  firm  fibrous  membrane — the  perichondrium.  This  membrane  is  well 
supplied  with  blood-vessels  and  nerves,  and  is  essential  to  the  growth  and 
maintenance  of  the  cartilage.  There  are  three  varieties  of  cartilage — 
namely,  hyaline  cartilage,  elastic  cartilage,  and  fibrocartilage. 

Hyaline  cartilage  is  of  a  faint  pearly-blue  color,  slightly  transparent, 
and  is  found  investing  the  articular  ends  of  the  bones — for  example, 
the  condyles  of  the  mandible;  also  forming  the  costal  and  nasal  carti- 
lages, as  well  as  those  of  the  trachea,  bronchi,  and  a  part  of  the  larynx. 
Hyaline  cartilage  is  distinguished  by  a  granular  or  homogeneous  matrix. 
The  cells,  which  contain  a  nucleus  with  nucleoli,  are  usually  grouped 
together  in  patches,  and  are  somewhat  irregular  in  outline,  appearing 
flattened  near  the  free  surface  of  the  tissue  in  which  they  are  placed,  and 
inclined  to  be  perpendicular  to  the  surface  in  the  more  deeply  seated 
portions.  The  matrix  is  dimly  granular  in  appearance,  resembling 
ground  glass,  and  receiving  its  name  from  this  fact.  That  part  of  the 
cartilage  close  to  the  perichondrium  is  supplied  with  cells  much  smaller 
than  those  occupying  the  lacunae  in  the  substance  of  the  mass,  and  the 
growth  of  the  cartilage  is  most  active  in  this  part.  Lining  each  lacuna 
is  a  delicate  membrane  (the  capsule),  which  primarily  is  but  partly  filled 
out,  but  as  the  cell  or  cells  increase  in  size,  this  membrane  is  carried  to 
the  walls  of  the  lacuna.  Articular  hyaline  cartilage  is  non-vascular, 
being  nourished  by  the  blood-vessels  of  the  bone  beneath. 

Elastic  cartilage  is  of  a  dull-yellow  color,  and  is  sometimes  called  yellow 
cartilage.  It  is  not  present  in  the  mouth,  but  occurs  in  the  external 
ear,  in  the  epiglottis,  and  in  part  of  the  larynx.  Its  structural  compo- 
sition is  quite  similar  to  hyaline  cartilage,  but  may  be  distinguished 
from  it  by  a  network  of  fine  elastic  fibers  which  penetrate  the  matrix. 
The  cells  are  rounded  or  oval,  containing  nuclei  and  nucleoli. 

Fibrocartilage  is  yellowish  or  milky  white  in  color,  and  is  much  more 
widely  distributed  throughout  the  body  than  the  elastic  variety.  It  is 
present  in  the  temporomandibular  articulation.  Like  those  previously 
described,  it  is  composed  of  cells  and  a  matrix,  the  latter  being  made  up 
of  fibrous  connective  tissue  arranged  in  bundles,  and  for  this  reason  it 
is  scarcely  deserving  the  name  of  cartilage,  only  that  in  other  portions 
continuous  with  it  cartilage-cells  may  be  found  in  abundance.  Between 
the  strata  of  the  fibrous  bundles  are  numerous  nucleated  cells,  which  are 
oval  and  more  or  less  flattened,  and  each  enveloped  in  a  delicate  capsule. 


BONE 


255 


Cartilage  is  further  classified  into  two  divisions — temporary  and 
permanent — the  former  term  being  applied  to  that  kind  of  cartilage  which 
in  the  fetus  and  in  youth  is  destined  to  be  converted  into  bone  (for  example, 
Meckel's  cartilage);  the  latter  class  including  all  those  cartilages  which 
are  generated  as  such,  and  continue  to  serve  in  that  capacity.  Temporary 
cartilage  closely  resembles  the  hyaline  variety,  being  formed  of  a  matrix 
in  the  lacunae  of  which  the  cells  are  located.  These  cells,  however,  are  not 
grouped  together  as  in  hyaline  cartilage,  but  are  more  uniformly  distrib- 
uted throughout  the  matrix. 

Bone. — Bone  is  mainly  composed  of  tricalcium  phosphate  and  cartil- 
age.    The  matrix  of  osseous  tissue  has  a  distinguishing  feature  produced 


Calcified 
Matrix 


Center~of  Calcifica- 
tion 


Fig.  18S. — Developing  Bone.      X  40. 


by  the  blending  of  organic  and  inorganic  substances,  resulting  in  hard- 
ness, solidity,  and  elasticity.  The  combination  of  organic  and  inor- 
ganic elements  in  bone  is  of  such  a  nature  that  either  part  may  be  removed 
without  destroying  the  other.  The  matrix  is  composed  of  the  salts 
of  lime,  especially  calcium  phosphate,  and  of  slender  fibrils  united 
by  a  cement-substance  into  bundles  of  various  sizes.  The  cement-sub- 
stance is  chiefly  composed  of  insoluble  lime-salts,  principally  carbonates 
and  phosphates.  These  two  kinds  of  structure  are  found  to  be  present 
in  different  parts  of  the  same  bone,  forming  a  dense  or  compact,  and  a 


256  HISTOLOGY 

spongy  or  cancellated  tissue.  The  former  occur  in  the  shaft  of  long 
bones  and  in  the  outer  layer  of  flat  or  irregularly  formed  bones.  Cancel- 
lated bone-substance  occurs  in  the  extremities  of  the  long  bones  and  in  the 
interior  of  flat  and  irregular  bones.  The  irregularly  formed  maxillary 
bones  give  place  to  both  kinds  of  bony  structure;  the  external  layer  of 
the  superior  maxillae  and  the  body  and  rami  of  the  inferior  maxilla  are 
composed  of  compact  tissue,  while  the  interior  of  these  bones  and  the 
condyloid  processes  of  the  mandible  are  spongy  or  cancellated  in  their 
nature.     When  examined  by  the  microscope  the  bony  substance  is  found 

:*3.  \ 


-;.  £>   I  ->r>x 


.».  »   .         ■  *^  '■  ■  f  o.   .-  -  —  -*■  <s    1  *■* 

Concentric  JHHH  \v"    "'*      r-' .  *V       .'     -■     " 

Lamella    j  -j^-..—'"""        »  -■:       -Jl'   ~~      ."-  "   r   - 

.       '    ■  -        *•  -  *  -       -  <•    ,  «•  •  ~>    ->.      a 

,  r  „    ^"*  /  -  V    -  "*-    H^j     -      ££        -  „        'x  ^ 

I  1 . 1  ■■'■';  1 !."    _    .  ^-  .'-•-.         V      >       ""*   '       - *  v       »        ■* 

Canal      ^'s""-  —    "3*  t  »    '    V.      . .; '       _-  1 .  ■    j  : ,  v.  Haversian 


Canal 


Fig.   189. — Transverse  Section  though  Shaft  of  Long  Bone.      X  30. 

occupied  by  numerous  little  spindle-shaped  spaces — lacuna.  Branching 
out  from  these  in  various  directions  are  minute  canals — canaliculi — 
which  anastomose  with  similar  canals  from  neighboring  lacunae.  In  the 
maxillary  bones  no  other  canals  than  these  may  be  visible,  but  if  a  trans- 
verse section  be  cut  through  one  of  the  long  bones,  an  additional  space 
makes  its  appearance  (Fig.  189) 

These  spaces  are  known  as  the  Haversian  canals.  They  are  circular 
in  outline  and  appear  as  a  center  for  a  small,  circular  district  mapped  out 
by  concentric  layers,  the  lacunas  and  canaliculi  following  the  same  con- 
centric plan,  and  through  each  other  communicating  with  the  Haver- 
sian canals.     The  general  direction  of  the  Haversian  canals  is  longitu- 


BONE  257 

dinal  with  the  long  axis  of  the  long  bones,  and  in  the  flat  or  irregular- 
shaped  bones  they  are  somewhat  irregular  in  formation  and  ramify  in 
various  directions.  In  the  osseous  matrix  each  lacuna  contains  a  bone- 
cell.  These  are  nucleated,  protoplasmic  cells.  In  developing  bone, 
these  cells,  which  do  not  completely  fill  the  lacunae,  are  connected  by 
numerous  branches  or  processes  passing  through  the  canaliculi;  in  older 
bone  very  few  processes  are  observed. 

There  are  two  processes  by  which  bone  may  be  prepared  for  histo- 
logical examination,  by  one  method  which  results  in  the  destruction  of 


Lacuna 


Haversian 
Canal 


:  .   A   !  ' 

Fig.  190. — Longitudinal  Section  of  Long  Bone.     X  30. 

the  organic  elements,  or  by  another  which  removes  the  inorganic  ele- 
ments. In  the  former  process  the  organic  matter  is  removed  by  simply 
drying  the  structures,  after  which  thin  sections  may  be  prepared  and  care- 
fully examined  under  the  microscope,  when  the  Haversian  canals,  lacunae, 
and  canaliculi  will  be  seen  forming  a  complete  concentric  network.  In 
the  latter  method  the  inorganic  substance  is  removed  by  immersing  a 
fresh  bone  in  dilute  picric  acid,  C6H2(N02)3OH,  which  readily  decalcifies 
it,  and  when  properly  prepared  sections  are  placed  under  the  microscope 
the  organic  contents  of  the  lacunae  and  canaliculi  alone  are  visible. 

The  concentric  laminae  of  bone  is  riveted  together  by  numerous 
delicate  rods  of  processes  named  Sharpey's  fibers,  these  delicate  fibers 
passing  through  the  'aminae  to  perform  this  office. 


258  HISTOLOGY 

Periosteum  and  Bone-marrow. — The  interstices  of  spongy  bone 
are  filled  with  a  soft  mass — the  bone-marrow — and  the  external  surface 
of  the  bone  is  covered  by  a  fibrous  membrane — the  periosteum.  This 
membrane  is  absent  where  bones  are  joined  to  each  other  by  ligament 
or  cartilage,  and  over  articular  surfaces.  The  periosteum  is  a  compact 
connective-tissue  membrane.  It  consists  of  two  layers:  an  outer,  fibrous 
layer  rich  in  blood-vessels,  which  forms  the  connection  with  adjacent 
structures;  an  inner  or  osteogenetic  layer  containing  few  blood-vessels, 
loose  in  texture,  but  rich  in  elastic  fibers  and  spheric  connective-tissue 
cells,  with  oval  nuclei.  These  are  the  formative  cells  of  bone  and  are 
called  osteoblasts.  These  cells  appear  in  the  lower  strata  of  the  inner 
layer,  or  the  layer  in  contact  with  the  bone,  and  are  especially  numerous 
during  the  period  of  development.  Through  the  blood-vessels  of  the 
bone  the  marrow,  internally,  is  placed  in  communication  with  the  perios- 
teum externally;  small  branches  given  off  from  the  numerous  arteries  and 
veins  of  the  periosteum  enter  the  Haversian  canals,  upon  which  they  pass 
to  the  canaliculi,  thus  communicating  with  the  blood-vessels  of  the 
marrow.  In  like  manner  numerous  nerves  enter  the  substance  of  the 
bone,  first  passing  into  the  Haversian  canals,  after  which  they  become 
closely  associated  with  the  minute  blood-vessels  and  are  distributed 
to  the  periosteum  and  bone-marrow.  The  bone-marrow,  besides  fill- 
ing the  interstices  of  the  spongy  substance,  is  also  found  occupying 
the  central  cavity  of  long  bones,  and  in  the  larger  Haversian  canals. 
The  marrow  is  of  two  varieties,  distinguished  by  its  color,  being  either 
red  or  yellow.  Red  marrow  is  found  in  the  flat  bones  (including  the 
maxillae),  the  vertebras,  and  ribs,  while  yellow  marrow  occurs  in  the 
long  bones  of  the  extremities.  Red  marrow  is  composed  of  a  delicate 
connective-tissue  network  supporting,  besides  the  marrow-cells,  a  few 
fat-cells  and  giant-cells.  In  the  long  bones  the  yellow  marrow  is  sur- 
rounded by  a  connective-tissue  membrane  lining  the  medullary  canals. 
Marrow-cells  and  giant-cells  are  present  in  abundance.  Marrow  is  very 
vascular  and  contains  many  osteoblasts. 

Dentin. — This  structure,  as  well  as  cementum,  which  in  many 
particulars  closely  resembles  bone,  will  be  fully  considered  in  connection 
with  the  histology  of  the  tissues  of  the  teeth. 

III.  MUSCULAR  TISSUE. 

Muscular  tissue  consists  of  elongated  or  fiber-cells  and  according  to 
the  structure  of  these  fibers  it  is  divided  into  three  classes — non-striated, 
striated  and  cardiac. 


MUSCULAR    TISSUE 


259 


a 


*  '   -  F 


fff 


-A 


«/ 


r 


1 


'  F 


Fig.  191. — A. — Longitudinal  section  of  smooth  muscle  fibers — a.  muscle  fiber;  b.  nucleus; 
c.  fibrous  tissue  between  fibers.  B. — Cross-section  of  smooth  muscle  fibers — a.  perimysial 
connective  tissue;  b.  blood-vessel;  c.  nucleated  fiber;  d.  nonnucleated  fiber.  C. — Longitudinal 
section  of  voluntary  muscle  fibers. — a.  sarcolemma;  b.  nucleus;  c.  end  of  muscle  fiber;  d.  dark 
bands;  e.  intermediate  disc;/,  nucleus;  g.  lateral  disc.  D. — Diagrammatic  section  of  cross 
and  long  striations — a.  dark  disc;  b.  lateral  discs;  c.  intermediate  disc.  E. — Cross-section  of 
voluntary  muscle — a.  perimysium;  b.  endomysium;  c,  nucleus  of  perimysium;  d.  fibrillre;  e. 
nucleus  of  muscle;  /,  sarcolemma.  F. — Longitudinal  section  of  cardiac  muscle  fibers — 
a.  muscle  fiber;  b.  nucleus;  c.  branch.  G. — Cross-section  of  cardiac  muscle  fibers — a.  peri- 
mysial sheath;  b.  nucleus  of  sheath;  c.  muscle  fiber;  d.  nucleus;  e.  radial  plates  of  fibrilke. 
(Radasch.) 


20O  HISTOLOGY 

Non-striated,  Smooth,  or  Involuntary  Muscular  Tissue. — This 
tissue  consists  of  contractile  fiber-cells  which  are  elongated,  spindle- 
shaped,  and  cylindric,  with  exceedingly  elongated  extremities,  which 
become  shorter  and  thicker  through  contraction.  They  are  quite  variable 
in  length  (i/io  to  1/450  of  an  inch),  and  are  composed  of  a  pale,  homo- 
geneous-looking protoplasm,  each  inclosing  an  elongated  or  rod-shaped 
nucleus,  which  is  flattened  if  the  cell  is  so  formed.  The  muscular  fibers 
are  firmly  bound  together  by  a  cement-substance,  forming  fasciculi, 
which  in  turn  are  collected  into  strata  or  membranes,  which  may  be  dis- 
posed parallel,  or  crossing  and  recrossing,  forming  an  intricate  network. 
The  connective-tissue  septa  provide  a  passageway  for  the  larger  blood- 
vessels, while  the  capillaries  penetrate  the  fasciculi  forming  a  compli- 
cated network  with  oblong  meshes.  Involuntary  muscular  tissue  is  not 
found  in  the  mouth  except  in  the  ducts  of  the  salivary  glands.  They  form 
the  main  constituent  part  of  the  middle  layer  or  coat  of  blood-vessels 
and  are  particularly  abundant  in  arteries. 

Striated  or  Voluntary  Muscular  Tissue. — Striated  muscular 
tissue  is  composed  of  long,  cylindric  fibers,  which  are  regularly  transversely 
striated.  In  most  instances  their  extremities  are  attached  to  bones  by 
means  of  tendons,  as,  for  example,  the  cheek-  and  lip-muscles.  The 
fibers  are  grouped  together  by  fibrous  connective  tissue  into  various  sized 
bundles,  forming  fasciculi.  There  is  much  variation  in  the  length  of  the 
fibers  composing  the  fasciculi  in  different  muscles.  In  most  instances 
the  fasciculi  which  serve  to  make  up  the  bundles  of  a  single  muscle  con- 
tinue parallel  with  one  another  throughout  their  length.  Surrounding  the 
whole  muscle  is  a  layer  of  connective  tissue  called  epimysium;  this  pene 
trates  between  the  individual  bundles  and  forms  a  covering  for  each  one 
of  them,  which  is  called  the  perimysium,  and  passing  from  this  into  the 
substance  of  the  bundle  is  a  stilt  more  delicate  connective  tissue,  the 
endomysium,  which  separates  the  individual  fibers  from  one  another. 
The  former  structure  carries  the  larger  blood-vessels  and  nerve-fibers, 
while  the  latter  supports  the  capillaries. 

Each  muscular  bundle  may  again  be  divided  into  smaller  bundles, 
which  in  turn  are  ensheathed  in  a  similar  manner  and  further  divisible, 
so  continuing  until  the  primitive  fasciculi,  or  so-called  muscular  fiber, 
is  reached.  Striped  muscular  fiber  consists  of  a  structureless,  elastic 
sheath,  the  sarcolemma,  which  structure  represents  the  cell-membrane, 
and  closely  invests  a  number  of  filaments  or  fibrils.  Besides  the  fibrillae, 
there  is  contained  within  this  fine,  structureless,  transparent  membrane 
the   sarcoplasm,   a   faintly   granular   substance   resembling   protoplasm, 


NERVOUS    TISSUES  261 

but  not  identical  with  it.  This  substance  serves  in  the  capacity  of  a 
matrix  for  the  fibrillar.  The  nucleus  is  found  beneath  the  sarcolemma. 
The  fibrillae  are  arranged  parallel  to  one  another,  being  supported  by 
the  sarcoplasm.  It  will  thus  be  seen  that  each  fiber  of  a  striated  muscle 
comprises  the  sarcolemma,  the  muscle-nuclei,  the  fibrillae,  and,  finally, 
the  sarcoplasm,  filling  all  the  interstices,  first  between  the  fibrillae  of 
each  muscle-column,  between  the  columns  of  each  group,  and  between 
the  groups  themselves.  The  disposition  of  the  sarcoplasm  may  be  most 
favorably  studied  by  a  cross-section  through  the  fibers,  appearing  as 
a  delicate  but  clear  network,  within  the  meshes  of  which  are  the  muscle- 
columns.  Striated  muscular  fibers  are  usually  tapering  off  and  becom- 
ing thinner  toward  their  extremities.  In  rare  instances  they  are  branched 
at  their  ends.  This  condition  is  present  in  the  tongue,  the  extremities  of 
the  fibers  passing  transversely  into  the  oral  mucous  membrane,  where 
they  become  further  subdivided.  The  striated  or  voluntary  muscles 
make  up  the  muscular  tissues  of  the  lips,  cheeks,  tongue,  and  soft  palate. 
Cardiac  or  Involuntary  and  Striated  Muscular  Tissue. — This 
tissue  is  found  only  in  the  heart.  The  fibers  are  short,  striated  and  have 
the  nucleus  in  the  center. 


IV.  NERVOUS  TISSUES. 

Until  within  recent  times  it  has  been  stated  that  the  nervous  tissue 
consists  of  two  histologic  elements  known  as  nerve-cell  and  nerve-fiber ; 
that  these  two  elements  differed  not  only  in  their  mode  of  origin,  but  in 
their  structure  and  physiologic  endowments.  At  the  present  time  it 
is  believed  that  the  entire  nervous  system  consists  of  an  infinite  number 
of  definite  independent  morphologic  units,  which,  through  having  a  com- 
mon origin  and  a  similarity  of  structure,  have,  nevertheless,  different 
functions  in  different  parts  of  the  body.  This  neurologic  unit  has  been 
termed  the  neuron,  and,  as  represented  schematically  in  figure  192,  may  be 
said  to  consist  of:  First,  the  nerve-cell,  or  neurocyte;  second,  nerve- 
process,  or  axon;  third,  the  end-tufts,  or  terminal  branches.  Each  of 
these  three  main  portions  of  the  neuron  presents  a  variety  of  secondary 
features  which  are  related  to  their  functional  activities. 

The  Nerve-cells,  or  Neurocyte. — The  nerve-cells  are  found  in  the 
cortex  of  the  brain,  in  the  interior  of  the  spinal  cord,  in  the  various 
ganglia  of  the  cerebrospinal  and  sympathetic  nervous  systems,  and  in  the 
organs  of  special  sense.     All  neurocytes  are  the  modified  descendants  of 


262 


HISTOLOGY 


independent  oval  or  pear-shaped  cell 

16 


''■  Dendrites 


—  Collateral  Branch 


Fig.  192. — Diagram 
Stbhr's  ' 


.Terminal  Branches 


of  a  Neuron.     (From 
Histology.") 


s  (the  neuroblasts) ,  originating  from 
the  epithelial  cells  which  form  the 
medullary  tube.  The  neurocyte  is 
at  first  smooth,  devoid  of  processes, 
and  endowed  with  ameboid  move- 
ment. In  the  course  of  develop- 
ment the  cells  project  a  greater  or 
less  number  of  processes  and  as- 
sume a  variety  of  shapes  and  sizes, 
in  accordance  with  variations  in 
functions;  thus,  the  cells  may  be 
spheroid,  pyramidal,  spindle- 
shaped,  stellate,  etc.  The  body  of 
the  cell  consists  of  a  protoplasmic 
basis,  more  or  less  granular,  con- 
taining a  well-defined  nucleus  and 
nucleolus.  A  centrosoma  has  also 
been  found  in  the  nerve-cell  in 
many  situations.  There  is  no  evi- 
dence, however,  of  the  existence  of 
a  cell-membrane.  From  the  body 
of  the  neurocyte  there  arises  one 
or  more  protoplasmic  processes, 
which,  passing  outward  in  various 
directions,  divide  and  subdivide 
into  a  greater  or  less  number  of 
branches,  which  are  collectively 
known  as  dendrites  or  dendrons. 
The  ultimate  subdivisions  and 
terminations  of  a  dendrite,  though 
forming  an  intricafe  feltwork, 
always  end  free,  never  anastomos- 
ing with  one  another.  Arising  from 
the  cell-body,  the  dendrites  resemble 
in  appearance  and  structure  the 
cell-protoplasm,  or  cytoplasm.  In 
the  cortex  of  the  cerebrum  and  in 
the  cortex  0}  the  cerebellum  the 
dendrites  are  characterized  by 
short,    lateral     projections    known 


NERVOUS    TISSUES  263 

as  lateral  buds  or  gemmules,  which  impart  to  the  dendrite  a  feathery 
appearance. 

The  Axon,  or  Nerve-process. — The  axon  is  the  first  outgrowth  of  the 
protoplasm  of  the  neuroblast,  but  with  the  development  of  the  neurocyte 
it  becomes  so  differentiated  from  the  dendrites  that  it  can  be  readily 
distinguished  from  them.  It  usually  arises  from  a  cone-shaped  projection 
of  the  cell-body,  though  occasionally  it  arises  from  a  dendrite  itself.  It  is 
characterized  by  a  short,  regular  outline  and  a  hyaline  appearance.  The 
majority  of  the  cells,  especially  in  the  mammalia,  possess  but  one  axon, 
though  in  the  developing  ganglion-cells  of  the  spinal  nerves  two  distinct 
axons  are  present.  In  their  subsequent  development  the  two  axons  appear 
to  blend  together  to  form  but  a  single  axon,  which,  at  a  short  distance  from 
the  cell,  again  divides  into  two  branches,  which  pursue  opposite  directions, 
one  passing  directly  into  the  spinal  cord,  the  other  toward  the  periphery. 
The  axon  may  continue  as  an  individual  structure  for  an  indefinite  dis- 
tance, varying  from  a  few 'millimeters  to  too  cm.  In  the  former  instance 
the  axon,  at  a  distance  of  a  few  millimeters  from  the  cell,  breaks  up  into 
a  number  of  branches,  which  form  an  intricate  feltwork  in  the  neighbor- 
hood of  the  cell.  This  type  of  cell  is  not  widely  distributed,  being 
confined  largely  to  the  cerebellum.  In  its  course  the  axon,  more  espe- 
cially in  the  central  nervous  system,  gives  off  a  number  of  side-branches  or 
collaterals,  which  do  not  differ  from  the  axon  itself,  either  in  structure  or 
appearance.  The  axon  of  the  peripheral  nerves,  especially  the  spinal 
nerves,  are  devoid  of  collaterals  throughout  their  extent,  except,  perhaps, 
in  the  immediate  neighborhood  of  the  cell.  The  more  or  less  elongated 
axon  becomes  inclosed  at  a  short  distance  from  the  cell  with  a  thick 
layer  of  fatty  material,  forming  a  medulla  or  myelin,  inclosed  by  a  deli- 
cate cellular  sheath  (the  neurilemma),  and  thus  constitutes  what  is  com- 
monly known  as  a  medullated  nerve-fiber:  In  the  central  nervous  sys- 
tem the  neurilemma  is  frequently  wanting.  In  the  sympathetic  system 
the  myelin  is  wanting,  though  the  axon  is  inclosed  by  a  delicate  sheath 
resembling  the  neurilemma,  thus  constituting  a  non-medullated  nerve- 
fiber.     The  collateral  branches  are  provided  with  similar  investments. 

The  End  Tufts,  or  Arborizations. — Each  axon,  as  it  approaches 
its  final  termination,  breaks  up  into  a  number  of  branches,  which  vary 
in  complexity  and  appearance  in  different  regions.  They  are  always 
free  from  any  medullary  investment,  and  appear  to  be  formed  by  the 
splitting  of  the  axon  into  a  number  of  fine  filaments,  which  remain  inde- 
pendent of  one  another.  In  peripheral  organs,  as  muscles,  glands,  and 
blood-vessels,  the  tufts  are  in  direct  organic  connection.     In  the  cen- 


264 


HISTOLOGY 


tral  nervous  system  the  end-tufts  are  in  more  or  less  intimate  relation 
with  the  dendrites  of  other  neurons. 

Nerves. — Nerves  are  to  be  regarded,  therefore,  as  groups  of  axons, 


Fig.  193. — A.  Multipolar  cell  from  cerebral  cortex;  B.  multipolar  cell  from  spinal  cord; 
C.  pyramidal  cell  from  cerebral  cortex;  D.  unipolar  cell;  E.  bipolar  cell;  F.  cell  of  Purkinje, 
antler  cell;  G.  mossy  cell;  H.  spider  cell;  I.  cell  from  spinal  cord  of  an  ox,  showing  pigment 
granules;  K.  ganglion;  L.  sympathetic  or  amyelinated  fibers;  M.  longitudinal  section  of 
myelinated  nerve  fiber — a.  neurilemma;  b.  myelin  sheath;  c.  axis  cylinder;  d.  node  of  Ranvier; 
e.  nucleus;  N.  cross-section  of  osmicated  nerve  fibers;  O.  myelinated  nerve  fiber  of  a  guinea- 
pig  showing;  the  reticulum  P.  myelinated  nerve  fibers  of  a  toad,  showing  reticulum  (neuro- 
keratin); R.  motor  neuron,  showing  nerve  cell,  dendrites,  axis  cylinder  and  ending  of  latter 
in  a  muscle,  S.  cross-section  of  nerve  trunk.     (Radasch.) 

with  their  medullary  investments  connecting  the  peripheral  organ  with 
the  central  nervous  system. 

The  nerves  are  arranged  in  two  great  systems — the  cerebrospinal 


NERVES 


265 


and  the  sympathetic.  In  the  cerebrospinal  nerves  the  conducting 
media — the  nerve  fibers — are  arranged  in  parallel  or  interlacing  bundles, 
and  these  are  further  grouped  into  nerve-branches  or  nerve-trunks. 
The  bundles  are  connected  by  intervening  fibrous  connective  tissue 
(the  epineurium) ,  and  through  this  tissue  the  principal  blood-vessels 
ramify  to  supply  the  nerve-trunks,  together  with  a  plexus  of  lymphatics 
and  numerous  fat-cells  and  plasma-cells. 

The  size  of  the  nerve-bundles,  or  funiculi,  is  regulated  according 
to  the  size  and  number  of  nerve-fibers  which  they  contain.  Investing 
each  funiculus,  or  primary  bundle  of  nerve-fibers,  is  a  connective-tissue 
sheath — the  perineurium.  The  fibers  composing  this  sheath  are  arranged 
in  lamellae,  being  separated  from  one  another  by  lymph-spaces  vari- 
able in  size,  through  which  communication  is  afforded  the  lymphatics  of 
the  epineurium.  Within  the  bundles  the  nerve-fibers  are  held  together 
by  fibrous  connective-tissue — the  endoneurium.     The  epineurium  holds 


Epineuriurr 


Perineurium 


Endoneurium 


Perineurium 


Fig.   194. — Transverse  Section,  Bundles  of  Nerve-fibers,  Human  Median  Nerve.      X30. 


together  and  envelops  the  several  funiculi  of  the  nerve-trunk,  the  peri- 
neurium investing  each  funiculus,  or  primary  bundle  of  nerve-fibers, 
and  the  endoneurium  extending  among  and  around  the  individual  fibers. 
Nerve-fibers  are  divided  into  two  classes — which  classification  is  depen- 
dent upon  the  presence  or  absence  of  a  medullary  sheath  or  covering — 
into  the  medullated  or  white,  and  the  non-medidlaled  or  gray.  The 
medullary  sheath,  or  white  substance  of  Schwann,  is  a  bright,  fatty  sub- 
stance (the  myelin)  surrounding  the  axon,  or  axis-cylinder,  the  conducting 


266  HISTOLOGY 

or  central  part  of  a  nerve-fiber.  Between  the  medullary  sheath  and  the 
axis-cylinder  there  is  present  a  small  amount  of  albuminous  fluid.  Closely 
surrounding  the  medullary  sheath,  and  forming  the  outer  boundary 
of  the  nerve-fiber,  is  the  neurilemma,  or  sheath  of  Schwann.  Between 
this  delicate,  structureless  membrane  and  the  medulla  there  are  placed 
at  intervals  oblong  nuclei,  surrounded  by  protoplasm;  these  are  the 
nerve-corpuscles.  Besides  the  division  of  nerve-fiber  into  medullated 
and  non-medullated,  each  division  is  susceptible  of  further  subdivision, 
dependent  upon  the  presence  or  absence  of  the  neurilemma.  Non- 
medullated  nerve-fibers  without  a  neurilemma  are  composed  of  an  axis- 
cylinder  only;  they  are  cylindric  or  band-like  in  form,  transparent,  and 
show  faint,  longitudinal  striations.  Non-medullated  nerve-fibers  with  a 
neurilemma  are  composed  of  an  axis-cylinder  surrounded  by  a  neurilemma, 
and  are  homogeneous  throughout  their  extent. 

Medullated  nerve-fibers  are  those  which  are  partly,  but  never  entirely, 
invested  by  a  medullary  sheath.  They  may  or  may  not  possess  a  neuril- 
emma; in  the  former  instance  they  consist  of  an  axis-cylinder  and  a 
medullary  sheath  only.  The  axis-c>linder,  or  essential  part  of  the  nerve- 
fiber,  is  cylindric  or  band-like,  occasionally  exhibiting  a  delicate,  longitu- 
dinal striation,  which  appearance  is  due  to  its  being  composed  of  a  primi- 
tive fibrillae. 

The  nerve-cells  or  ganglia-cells  are  found  in  the  ganglia  as  well  as 
along  the  course  of  the  nerves.  They  are  composed  of  granular  or 
faintly  striated  protoplasm,  inclosing  a  characteristic  nucleus  within  which 
is  a  nucleolus.  They  differ  greatly  in  form  as  well  as  in  size,  the  spheric, 
spindle-shaped,  and  irregularly  stellate  forms  being  the  most  common. 
In  the  latter  numerous  processes  are  given  off,  forming  the  stellate  out- 
lines. The  cells  are  variously  named,  according  to  the  number  of  proc- 
esses. If  one  process  is  present,  the  cell  is  termed  a  unipolar  cell;  if 
two,  a  bipolar;  and  if  a  number  of  processes  exist,  they  are  named  multi- 
polar. The  processes  are  of  two  varieties — the  axis-cylinder  process 
and  the  branched  protoplasmic  process.  The  various  forms  are  most 
readily  distinguished  in  the  multipolar  cells.  The  axis-cylinder  process 
is  readily  characterized  by  its  hyaline  appearance  and  unbroken  outline. 
The  protoplasmic  processes  are  thicker,  granular,  and  striated. 

V.  BLOOD  AND  LYMPH. 

It  is  rather  difficult  to  conceive  that  blood  and  lymph  may  be  spoken  of 
as  representing  one  of  the  elementary  animal  tissues.  The  conception  of 
an  elemetary  tissue,  as  exemplified  in  the  four  tissues  just  considered, 


BLOOD   AND    LYMPH  267 

carries  with  it  the  idea  of  some  well-defined  stationary  structure.  Blood 
and  lymph  on  the  contrary,  present  fluids,  which  uninterruptedly  stream 
along  within  closed  channels — the  blood-vessels  and  lymph-vessels.  A 
closer  study  of  the  development  and  structure  of  blood  and  lymph,  how- 
ever, reveals  a  very  close  relation  and  great  similarity  of  their  cellular 
elements  to  those  of  the  other  elementary  tissues.  On  the  other  hand, 
we  must  take  into  consideration  that  blood  and  lymph  constitute  the 
medium  by  means  of  which  nutrient  and  building  material  is  carried  to 
all  remotest,  as  well  as  minutest  parts  of  the  body,  and  in  exchange  re- 
ceives and  carries  away  the  waste  products.  Now,  it  is  quite  obvious 
that  the  distribution  of  blood  and  lymph  throughout  the  body  can  only 
take  place  when  the  free  motion  of  its  constituent  parts  is  not  handicapped 
by  a  tenacious  or  solid  intercellular  substance.  It  can  only  be  accom- 
plished because  the  intercellular  substance  is  a  fluid.  If  a  quantity  of 
blood  is  drawn  and  exposed  to  the  air  it  coagulates,  which  means,  that  the 
fluid  intercellular  substance  becomes  fibrous,  and  as  the  density  gradually 
increases  its  real  similarity  to  other  elementary  tissues  becomes  very 
evident.  While,  thus,  blood  and  lymph  present  conspicuous  peculiarities, 
it  is  nevertheless  justifiable  to  consider  them  as  a  distinct  elementary 
tissue.  We  may  define  it  as:  "a  tissue  like  all  other  tissues,  consisting 
of  cells  and  intercellular  substance,  but  in  which  the  intercellular  substance 
is  a  fluid." 

Development. — Blood  and  lymph,  as  well  as  the  channels  in  which 
they  originate  in  the  mesoderm,  from  groups  of  cells  especially  prede- 
termined for  that  purpose.  Some  of  the  cells  become  joined  together  by 
means  of  their  protoplasmic  processes  in  a  chain-like  fashion,  and  grad- 
ually becoming  flattened,  form  continuous  epithelium-like  layers  known 
as  endothelium  or  mesoihelium,  which  ultimately  become  arranged  into 
the  shape  of  tubes.  Other  cells  of  the  same  group  do  not  exhibit  any  pro- 
cesses of  their  cytoplasm,  they  are  more  or  less  spherical  in  outline,  and 
while  the  formation  of  the  tubes  is  going  on,  these  cells  remain  separated 
from  one  another  and  become  imprisoned,  as  it  were,  within  the  latter. 
The  gradually  developing  tubes  are  the  future  blood-  and  lymph-vessels, 
and  the  cells  inclosed  within  the  latter  are  the  future  blood-  and  lymph- 
cells  or  corpuscles.  Between  the  individual  corpuscles  there  gradually 
forms  an  intercellular  substance,  which  becomes  differentiated  into  a 
fluid  known  as  blood-  and  lymph  plasm  and  thus  a  possibility  for  the  cells 
to  stream  along  within  closed  channels  and  fulfill  the  duties  assigned 
to  them  in  the  economy  of  the  body,  becomes  established. 

We  also  observe,  however,  that  many  of  the  cells  belonging  to  the  same 


268 


HISTOLOGY 


group  do  not  meet  the  fate  of  the  cells  just  described  but  retain  an  in- 
dependent individuality.  Some  of  them  become  endowed  with  a  vital 
characteristic  peculiar  to  one  of  the  lowest  unicellular  organisms — the 
ameba,  which  is  known  as  ameboid  motion.  Owing  to  this  faculty, 
these  cells  are  constantly  changing  their  place  and  travel  along  between 
the  cells  in  the  variously  differentiated  tissues,  and  are  therefore  known 
as  wandering  or  migratory  cells.     Some  of  the  migratory  cells  are  able 


M.T 


Fig.  195. — Section  from  the  Head  of  a  Rabbit  Embryo  of  Ten  and  One-half  Days,  4.4  mm., 
to  show  Mesenchyma.  (Slohr.)  Epi.  and  M.T.,  Ectodermal  epithelium  of  the  epidermis  and 
medullary  tube,  respectively.  N.,  Nucleus;  P.,  protoplasm;  and  I.S.,  intercellular  substance 
of  a  mesenchymal  cell.  Two  of  these  cells  show  mitotic  figures.  B.V.,  Blood-vessel,  lined  by 
endothelium.     One  of  the  blood-vessels  contains  an  embryonic  red  blood  corpuscle. 

to  incorporate  into  their  cytoplasm  various  substances  of  inorganic  or 
organic  nature,  as  well  as  whole  living  micro-organisms.  They  may 
either  digest  them  or  in  traveling  along,  carry  these  substances  and  in- 
cidently  deposit  them  in  other  localities,  sometimes  quite  remote  from  the 
place  where  they  have  been  picked  up.  These  cells  are  known  as  phago- 
cytes, and  are  of  great  significance  in  the  body  economy. 

Some  of  these  migratory  cells  are  identical  with  the  cellular  elements 
found  in  the  lymph,  and  are  therefore  known  as  lymphoid  cells.  These 
cells  are  found  distributed  throughout  the  body  and,  in  various  localities 


BLOOD   AND    LYMPH  269 

they  aggregate  into  larger  masses  and  form  the  so-called  lymphoid  tissue 
and  lymphoid  organs,  of  which  we  will  speak  later. 

Structure. — We  have  to  keep  in  mind  that  the  ultimate  role  of  blood 
and  lymph  is  to  keep  up  the  interchange  of  substances  in  the  body,  which 
in  a  general  way  takes  place  as  follows:  The  lymph  absorbs  the  ultimate 
products  of  digestion,  and  delivers  them,  through  the  thoracic  duct,  to 
the  blood.  Laden  with  these  substances,  the  blood  passes  through  the 
lungs  and  here  it  gives  off  the  deleterious  C02  to  the  atmosphere  and 
receives  in  exchange  a  corresponding  amount  of  O.  It  then  passes  to 
the  heart,  and  through  the  activity  of  that  organ  it  becomes  distributed 
throughout  the  body.  Now,  just  as  the  function  of  the  blood  differs  from 
the  function  of  the  lymph  so  these  two  tissues  differ  in  their  structure. 
The  active  elements  in  the  process  of  absorption  taking  place  in  the 
lymph  are  the  above-  described  migratory  cells.  They  constitute  the 
cellular  elements  of  the  lymph  and  are  therefore  known  under  the  common 
name  of  lymphocytes.  The  active  elements  in  the  process  of  exchange  of 
gases  taking  place  in  the  blood  are  cells  containing  a  chemically  active 
substance  hemoglobin,  which  is  the  cause  of  the  red  color  of  the  blood, 
and  these  cells  are  therefore  known  as  the  red  corpuscles  or  erythrocytes. 
In  view  of  the  fact  however,  that  the  lymph,  as  stated  above,  ultimately 
enters  and  becomes  a  component  part  of  the  blood  it  is  obvious  that  the 
blood  must  contain  the  active  elements  of  both,  and  we  actually  find  in 
the  blood  two  kinds  of  cellular  elements,  which  are  correspondingly 
called:  (a)  colored  or  red  blood  corpuscles  or  erythrocytes  and  (b)  colorless 
or  white  blood-corpuscles  or  leucocytes.  The  average  number  of  these 
per  cubic  millimeter  is  5,000,000  red  corpuscles  to  8,000-10,000  white 
corpuscles,  making  a  ratio  of  500  :i. 

Erythrocytes. — The  red  blood-corpuscles  are  usually  described  as 
colored,  non-nucleated,  biconcave,  circular  discs,  of  7.5-8  micr.  or 
1/3200  inch,  in  diameter.  When  blood  is  at  rest,  the  corpuscles  show 
a  decided  tendency  to  stick  together  in  clumps  or  be  piled  up  in  columns 
known  as  rouleaux.  The  main  characteristics  of  the  erythrocites  have 
always  been,  and  still  continue  to  be,  a  matter  of  investigation  and  dis- 
cussion, and  the  following  points  must  be  noted. 

The  color  is  due  to  the  iron-containing  chemical  compound,  which  is 
known  as  Hemoglobin.  When  examined  singly,  the  corpuscles  have  a 
rather  greenish-yellow  tint,  but  when  aggregated  in  larger  quantities,  and 
suspended  in  their  natural  medium,  the  blood-plasm,  they  appear  red. 

In  regard  to  shape,  there  is  hardly  any  doubt  that  the  form  of  circular 
biconcave  disks  is   greatly   predominating.     Some,   however,    present   a 


270 


HISTOLOGY 


cup-shape;  we  also  see  some  of  a  spherical  or  globular  shape,  and  a  very 
conspicuous  form  is  the  one  known  as  crenated.  This  variety  of  forms  of 
the  red  corpuscles  can  be  explained  without  difficulty  if  we  take  into 
consideration  the  condition  of  the  fluid  in  which  they  are  suspended.  It 
is  obvious  that  when  the  density  of  the  plasm  is  decreased  the  corpuscles 
will  become  more  permeated  with  the  fluid  and  will  expand.  This  ex- 
pansion may  cause  a  bulging  out  only  on  one  side  of  the  disc  and  will 
produce  a  cup-shape,  or  it  may  cause  a  bulging  out  on  both  sides  and  the 
globular  form  will  be  the  result.     On  the  other  hand,  if  the  density  of  the 


Fig.  196. — Red  Corpuscles  in  Various  Conditions.  (Stohr.)  A,  Biconcave  disc  as  seen 
with  objective  lowered  and  C,  with  objective  raised  B,  Seen  on  edge.  D,  Cup-shaped  form. 
E,  Irregular  contractions  and  distortions.  F,  Corpuscles  from  which  the  hemoglobin  is  re- 
moved.    (Shadows.)     G,  Crenated  form.     H,  Extrusions  from  the  corpuscle. 

plasm  is  increased,  by  evaporation  for  example,  the  surface  tension  of  the 
corpuscles  will  be  decreased,  an  irregular  shrinkage  will  take  place  and 
the  crenated  form  will  result. 

In  regard  to  the  nucleus,  we  have  to  state  that,  while  the  colored 
blood-corpuscles  of  lower  animals  are  nucleated,  those  of  human  blood 
and  blood  of  other  mammals  are,  when  examined  in  the  adult,  always 
non-nucleated.  This  fact  made  it  rather  doubtful,  whether  these  cor- 
puscles can  be  considered  as  cells,  as  it  is  not  compatable  with  our  defini- 
tion of  a  cell  as:  a  mass  of  protoplasm  containing  a  nucleus.  The  study 
of  the  development  of  the  blood  has  however  entirely  cleared  up  that 
point.  We  have  stated  above,  that  blood  and  lymph,  as  well  as  the 
channels  in  which  it  flows,  originate  in  the  mesoderm,  from  cells  specially 
destined  for  that  purpose,  which  are  therefore  known  as  angioblasts. 
Those  of  the  angioblasts,  which  later  become  the  cellular  elements  of 
the  blood  are  known  as  hematoblasts  and  are  like  the  rest  of  the  mesoderm 
cells,  nucleated.  A  number  of  these  hematoblasts  become  gradually 
colored  through  formation  of  hemoglobin  within  the  cells  and  are  then 
known  as  erythroblasts.  These  cells  still  contain  a  nucleus,  but  gradually 
the  latter  disappears  and  they  become  moulded  into  the  well-known 
erythrocytes.     The  exact  process  of  the  disappearing  of  the  nucleus  is  not 


BLOOD   AND    LYMPH  27 I 

positively  established  as  yet;  some  investigators  are  of  the  opinion  that 
the  nucleus  becomes  extruded  from  the  cell  and  may  be  found  as  a  dis- 
tinct small  corpuscle  within  the  blood,  others  think  that  the  nucleus 
becomes  entirely  dissolved  and  no  traces  of  it  are  left.  Be  that  as  it 
may,  tjje  fact  established  beyond  doubt  is,  that  all  red  corpuscles  possess  a 
nucleus  in  the  early  stages  of  their  life-history  and  therefore  must  be  con- 
sidered as  true  cells,  which  have  undergone  important  modification. 
Deprived  of  the  nucleus,  the  red  blood-corpuscles  have  lost  their  ability 
to  produce  new  cells,  and  consequently  new  corpuscles  can  only  be  sup- 
plied by  gradual  transformation  from  undifferentiated  hematoblasts. 
This  process  takes  place  throughout  the  entire  life  of  the  organism, 
mainly  in  the  bone-marrow,  and  it  is  in  this  locality  where  we  can  observe 
and  study  the  erythrocytes  in  all  stages  of  their  development. 

Leucocytes. — The  white  blood-corpuscles  are  colorless  spherical  bodies, 
which  are  constantly  present  in  blood,  but  owing  to  the  fact  that  they  are 
colorless  and  their  number  is  very  much  smaller  than  the  number  of  the  red, 
they  are  not  so  easily  detected.  These  corpuscles  present  all  attributes 
of  true  cells  and  have  always  been  considered  as  such.  We  can  distin- 
guish in  them  a  cytoplasm  and  a  nucleus,  and  a  great  majority  of  them 
manifest  ameboid  movement.  Due  to  the  latter  faculty  they  can  make 
their  way  out  or  in  through  the  thin  walls  of  the  fine  blood  channels  and 
creep  about  as  wandering  cells  among  other  cells  of  the  tissues.  During 
their  excursions  some  may  incorporate  various  particles  which  they  are 
liable  to  meet  with  and  also  extrude  them  here  and  there  on  the  route 
which  they  cover,  thus  acting  as  phagocytes. 

Within  the  last  few  decades,  the  migratory  activity  of  the  leucocytes 
and  their  phagocytic  action  became  to  be  considered  as  one  of  the  most 
important  factors  in  carrying  and  transmitting  disease-producing  germs. 
They  are  also  considered  as  factors  of  great  importance  in  curing,  as  well 
as  preventing,  diseases,  by  holding  up  these  germs,  destroying  them  and 
removing  the  debris  from  the  body.  It  is  due  to  this  fact  mainly,  that 
the  white  blood-corpuscles  have  attracted  the  attention  of  many  distin- 
guished investigators,  and  many  interesting  and  important  facts  have  been 
revealed.  It  was  in  these  studies  particularly  that  the  great  importance  of 
using  various  dyes  for  microscopic  examinations  has  been  very  clearly 
demonstrated,  since  it  was  the  application  of  the  various  stains  which  led 
to  the  most  important  discoveries  in  this  domain.  We  know  at  present, 
that  the  cells  of  the  blood  which  are  known  collectively  as  leucocytes,  are 
not  all  of  the  same  kind.  They  differ  in  size  as  well  as  in  structure  and 
the  following  classifications  and  nomenclature  is  in  vogue  at  present. 


272  HISTOLOGY 

(1)  One  classification  is  based  upon  the  size  of  the  cells  and  the 
character  of  the  nucleus,  and  there  can  be  distinguished  four  varieties 
of  cells  from  this  point  of  view. 

(a)  Small  Mononuclear  Leucocytes. — As  the  name  indicates,  these  cells 
contain  only  one  nucleus  and  are  of  small  size.  They  are  also  known  as 
small  lymphocytes,  and  constitute  about  20  per  cent,  of  all  leucocytes  in 
the  blood. 

(b)  Large  Mononuclear  Leucocytes. — They  are  also  known  as  large 
lymphocytes,  and  their  number  is  only  about  1  per  cent. 

(c)  Polynuclear  or  Polymorphonuclear  Leucocytes. — As  the  name  indi- 
cates, these  cells  have  several  nuclei,  and  the  shape  of  the  latter  varies. 
This  is  the  predominating  form  among  the  leucocytes  of  the  blood,  and 
constitutes  about  70  per  cent. 

(d)  Transitional  Leucocytes. — Under  this  heading  are  grouped  leuco- 
cytes with  a  structure  somewhat  midway  between  the  others.  They  con- 
stitute about  7  per  cent. 

(2)  Another  classification  is  based  upon  the  presence  or  absence  of 
various  granules  in  the  cytoplasm.  These  granules  can  be  distinguished 
only  when  various  stains  are  applied  and  manifest  a  difference  in  their 
affinity  to  the  various  dyes.  This  method  of  studying  blood  for  leuco- 
cytes constitutes  at  present  every-day  work,  we  might  say,  of  a  clinical 
examination.  There  have  been  many  forms  of  leucocytes  described  and 
quite  a  number  of  names  are  used.  It  is  beyond  the  scope  of  this  book 
to  describe  them  all,  but  we  think  that  it  is  necessary  that  the  readers 
should  be  familiar  with  the  principal  points  of  this  subject.  To  under- 
stand the  classification  given  below  it  is  necessary  to  keep  in  mind  the 
following  point.  The  anilin  dyes  used  for  microscopic  examination  are 
procured  in  form  of  powders,  which  present  chemical  compounds  known 
as  salts  and  are  soluble  in  water  or  alcohol  or  in  both.  Like  other 
salts,  they  consist  of  a  base  and  an  acid  and  it  has  been  found  that  in 
some  dyes  the  basic  principle  has  the  staining  qualities,  in  others  the  acid 
principle  has  the  staining  qualities,  while  in  a  third  group  the  staining 
qualities  are  due  to  both.  We  therefore  distinguish  three  kinds  of  stains: 
(a)  basic  stains,  (b)  acid  stains,  (c)  neutral  stains.  On  the  other  hand  it 
has  been  conclusively  demonstrated  that  some  of  the  granules  in  the  cy- 
toplasm are  stainable  with  basic  stains,  and  these  are  therefore  called 
basophilic  granules,  others  are  stainable  with  acid  stains  and  are  there- 
fore known  as  acidophilic  granules,  and  still  others  are  stainable  with 
both  and  are  known  as  neutrophilic  granules.  With  these  facts  kept  in 
mind,  the  following  classification  will  be  conceived  without  difficulties. 


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Fig.  197. — -Chief  varieties  of  cells  encountered  in  health  and  disease  (Wright's  stain).  1. 
Normal  red  cell.  2.  Common  form  of  polymorphonuclear  leucocyte.  3.  Lesser  lympho- 
cyte. 4.  Eosinophilic  myelocyte.  5.  Eosinophilic  leucocyte.  6-6.  Neutrophilic  leucocytes: 
upper  left,  transitional  form,  on  right  neutrophilic  myelocytes.  7-7.  Large  lymphocytes. 
8.  Normoblast  (angioblasts) .  8.  Normoblast  showing  division  of  nucleus,  o.  Normo- 
blast nucleus.  10-11.  Basophilic  leucocytes.  12.  Megaloblast.  (Greene's  Medical  Diag- 
nosis.) 


BLOOD   AND    LYMPH  273 

(a)  Acidophiles. — Leucocytes  with  granules  stainable  with  acid 
stains.  They  are  also  known  as  Eosinophiles,  because  the  stain  used 
when  they  were  first  described  was  eosin. 

(b)  Basophiles.  (Coarse). — They  are  also  called  "mast  cells." 

(c)  Basophiles.  (Fine). — They  are  identical  with  the  large  mononu- 
clear leucocytes. 

(d)  Neutrophils. — They  are  identical  with  the  polymorphonuclear 
leucocytes. 

It  is  now  a  very  well-established  fact  that  various  diseases  are  char- 
acterised by  the  predominance  in  the  blood  of  the  patients,  of  one  or  the 
other  of  the  various  forms  of  leucocytes.  It  has  also  been  found  that  in 
the  same  disease  the  character  of  the  leucocytes  as  a  predominating 
feature  may  differ  during  the  various  stages  of  the  disease.  These  facts 
are  undoubtedly  of  great  significance,  but  the  causes  underlying  them  are 
still  a  mystery. 

Blood-platelets.— Besides  the  erythrocytes  and  leucocytes,  there  has 
been  observed  in  the  blood  a  third  form  of  corpuscles,  which  are  known 
as  blood-platelets.  These  are  very  minute  circular  discs,  which  are  con- 
sidered to  be  of  importance  as  factors  in  blood  coagulation,  and  are  there- 
fore also  called  thrombocytes.  In  regard  to  their  structure  the  statements 
of  the  various  investigators  differ  very  materially,  and  also  in  regard  to 
their  origin  many  views  have  been  expressed.  While  some  investigators 
consider  the  platelets  as  distinct  independent  cellular  elements,  others 
look  upon  them  as  disintegrated  fragments  of  erythrocytes  or  of  leuco- 
cytes. Some  see  in  them  the  expelled  nuclei  of  erythrocytes,  others  con- 
sider them  as  constricted  and  discarded  protoplasmic  parts  of  leucocytes. 
We  must  say,  that  notwithstanding  the  great  amount  of  research  work 
done  and  quite  considerable  literature  accumulated,  the  mystery  surround- 
ing these  corpuscles  still  remains  unsolved. 


CHAPTER  III. 

Circulatory  Organs,  Glands. 

In  the  previous  chapters  we  have  described  the  elementary  tissues 
and  have  pointed  out  that  each  one  of  these  tissues  is  differentiated  to  per- 
form a  well-defined  function  in  the  economy  of  the  organism  as  a  whole. 
The  functions  of  the  individual  tissues  however  become  manifest  only  when 
two  or  more  of  the  elementary  tissues  are  associated  to  form  more  or 
less  complex  units  known  as  organs.  There  are  a  number  of  organs  in 
which  dentists  are  more  or  less  interested,  as  these  organs  either  form 
the  wall  of  the  mouth  cavity,  or  are  situated  within  the  mouth  cavity, 
or  in  close  proximity  to  it.  As  regards  their  gross  anatomical  relations, 
these  organs  have  been  considered  in  the  first  part  of  this  book  and  we 
are  now  ready  to  study  their  minute  anatomy. 

ORGANS  OF  CIRCULATION. 

The  organs  of  circulation  consist  of  a  system  of  tubes  of  various  sizes 
known  as  blood-vessels,  and  a  central  apparatus  which  furnishes  the 
motive  power  for  propelling  the  blood  within  these  tubes — the  Heart. 

If  we  consider,  for  example,  the  blood  circulation  in  the  mandible, 
we  find  that  the  blood  starting  from  the  heart  takes  its  course  first  to  the 
aortic  arch.  Here  it  is  distributed  among  vessels  of  smaller  caliber, 
one  of  these  being  the  common  carotid  artery.  From  this  channel,  the 
blood  is  again  distributed  into  arteries  of  still  smaller  diameter,  and  a 
part  of  it  enters  the  inferior  dental  artery  which  also  branches  off  into  a 
number  of  still  smaller  vessels  which  conduct  the  blood  to  the  pulps  of 
the  individual  teeth.  Here  we  find  the  blood  circulating  in  a  network 
of  fine  vessels  known  as  capillaries.  The  walls  of  these  capillaries  are 
very  thin  and  therefore  a  close  contact  of  the  blood  with  the  tissues  takes 
place.  Becoming  laden  with  the  waste  materials  of  the  tissues,  the  blood 
continues  its  journey  through  blood-vessels  known  as  veins,  which  grad- 
ually increase  in  size.  Finally  it  enters  the  vena  cava  inferior,  and  is 
thus  brought  back  to  the  heart. 

Of  the  three  kinds  of  blood-vessels  mentioned;  the  first, 

274 


ORGANS    OF    CIRCULATION 


275 


i.  Arteries  carry  the  blood  to  the  tissues;  the  second, 

2.  Veins  carry  the  blood  back  from  the  tissues,  while  the  third, 

3.  Capillaries  distribute  the  blood  among  the  tissues  and  thus  become 
instrumental  in  performing  the  function  assigned  to  the  blood  in  the 
economy  of  the  body. 


Fig.  198. — Blood-vessels  from  a  Rabbit  Embryo  of  Thirteen  Days,  developing  as  Endo- 
thelial Sprouts  (en)  from  pre-existing  vessels  (bv) ;  b.c,  blood  corpuscle  within  a  vessel.   (Stohr.) 

Conforming  to  their  functions,  the  three  kinds  of  blood-vessels  show 
marked  differences  in  their  structure,  and  the  understanding  of  this 
subject  may  be  facilitated  by  referring  to  the  mode  of  development. 

We  have  stated  before  (p.  267)  that  some  cells  in  the  mesoderm  become 
arranged  in  a  simple  squamous  epithelium-like  fashion,  known  as  endo- 


Fig.  199. — Small  Arteries  of  Man.  (Stohr.)  Nuclei  of  endothelial  cells;  m,  nuclei  of 
circular  muscle  fibers,  at  -m'  seen  in  optical  cross-section;  a,  nuclei  of  connective  tissue.  In 
A,  since  the  endothelium  is  out  of  focus,  its  nuclei  are  not  seen.      X240. 

(helium,  and  form  tubes  consisting  of  a  single  layer  of  endothelium  or 
mesothelium.  The  walls  of  the  capillaries  consist  practically  only  of  a 
single  layer  of  endothelial  cells,  but  as  the  caliber  of  the  tubes  increases 
the  walls  become  strengthened  with  a  few  strands  of  connective  tissue. 
The  inner  lining  of  all  parts  of  the  circulatory  apparatus  show  the  same 


276 


HISTOLOGY 


kind  of  an  endothelial  lining,  and  the  differences  which  are  manifest,  are 
found  only  in  the  tissues  which  surround  this  lining  and  give  the  tube 
more  strength,  firmness  and  elasticity,  depending  on  the  size  of  the  blood- 
vessel, the  amount  of  blood  it  contains  and  the  degree  of  pressure  which 
it  exercises  upon  its  walls.  If  we  take  as  typical,  a  medium  sized  artery, 
we  generally  recognize  three  coats: 


Endothelium 


Intima 


Media 


Externa 


Smooth 
muscle  fibers 


Fig.  200. — Portion  of  a  Cross-section  of  the  Brachial  Artery  of  Man.     Xioo.     (Stohr.) 


i.  The  inner  coat  (intima)  which  consists  of  a  single  layer  of  endo- 
thelial cells  resting  upon  a  subendothelial  connective  tissue  and  a  strand 
of  elastic  tissue  known  as  the  inner  elastic  lamina. 

2.  The  middle  coat  (media)  which  consists  of  several  layers  of  smooth, 
short,  circularly  arranged  muscle  fibers  interwoven  with  connective- tissue 
fibers,  mostly  of  the  elastic  variety. 

3.  The  outer  coat  (adventitia)  which  consists  of  connective  tissue  which 
ultimately  blends  with  the  connective  tissue  surrounding  the  artery  as  a 
whole. 

Veins  have,  broadlv  speaking,  the  same  structure  as  arteries  but 
differences  are  shown  in  the  relative  quantity  of  one  or  the  other  elementary 


ORGANS    OF    CIRCULATION 


277 


2, 


Endothelium 


Inner  elastic 
membrane 


Connective 
tissue 


Nuclei  of 

smooth 

muscle 

fibers 


Connective 
tissue 


Fig.  201. — Part  of  a  Cross-section  of  a  Vein  from  a  Human  Limb.  X230.  (Slohr.} 
The  elastic  elements  are  shown  very  black.  1,  Intima;  2,  media;  3,  externa.  (The  middle 
of  the  three  objects  labled  nuclei  of  smooth  muscle  is  apparently  an  elastic  fiber.) 


mm/ 

Fig.  202. — Section  of  the  Heart.  (Stohr.)  ca,  Capillaries;  en,  endothelium;  La.,  left 
atrium;  I. v.,  left  ventricle;  mes.,  mesothelium  (of  the  epicardium,  or  visceral  pericardium); 
p.c,  pericardial  cavity;  p.p.,  parietal  pericardium;  r.a.,  right  atrium;  r.v.,  right  ventricle; 
si.,  sinusoids;  v.b.,  bicuspid  valve;  v.t.,  tricuspid  valve;  v.v.s.,  valve  of  the  venous  sinus. 


278  HISTOLOGY 

tissues.  In  a  medium-sized  vein,  we  generally  find  the  walls  thinner, 
and  the  inner  elastic  lamina  of  the  intima  is  much  thinner  or  is  entirely 
wanting.  The  media  shows  less  muscle  fiber  and  more  connective  tissue. 
The  adventitia  shows  the  same  structure  as  that  of  the  arteries,  but  is 
considerably  thicker. 

While  arteries  and  veins  constantly  have  blood  passing  through  them, 
their  tissues  are  not  nourished  from  this  source,  but  receive  the  blood 
supply  needed  for  their  metabolism  by  means  of  special  blood-vessels 
situated  within  the  outer  connective-tissue  'layer  {adventitia)  which  are 
known  as  vasa  vasorum. 

If  we  now  examine  the  structure  of  the  heart,  we  find  the  following: 

1. .  The  inner  lining  as  a  continuation  of  the  inner  lining  of  the  blood- 
vessels, consisting  of  a  layer  of  endothelial  cells  and  subendothelial  con- 
nective tissue  corresponding  to  the  intima  and  known  as  Endocardinm. 

2.  The  middle  coat  (myocardium)  which  forms  the  bulk  of  the  heart, 
and  consists  of  many  layers  of  muscular  tissue  running  in  various  direc- 
tions and  showing  a  special  structure  of  the  so-called  cardiac  muscle. 
•  3.  The  outer  coat  (epicardium)  shows  some  peculiarities,  inasmuch  as 
it  consists  of  a  layer  of  endothelial  cells  and  some  subendothelial  con- 
nective tissue  forming  the  so-called  Epicardium.  This  layer  when  it 
reaches  the  roots  of  blood-vessels,  turns  back  again  and  forms  the  so- 
called  Pericardium.  In  the  heart  and  in  a  number  of  veins,  the  endo- 
cardium bulges  out  in  the  form  of  folds  and  forms  the  valves.  In  regard 
to  the  nourishment  of  the  heart,  we  repeat  the  same  that  was  said 
in  regard  to  the  blood-vessels,  namely  that  while  all  the  blood  passes 
through  the  heart,  the  supply  of  blood  for  the  metabolism  of  the  heart 
tissue  itself,  is  carried  on  by  special  vessels,  the  so-called  coronary  arteries 
and  veins,  which  therefore  correspond  to  the  vasa  vasorum  of  the  blood- 
vessels and  are  located  in  a  position  corresponding  with  the  latter,  namely, 
the  connective  tissue  of  the  outer  layer,  the  epicardium,  beneath  the 
endothelium. 

GLANDS. 

A  group  of  organs  distributed  in  various  localities  throughout  the 
body  and  which  are  of  interest  to  the  dental  student  are  the  so-called 
glands.  Many  of  them  are  considered  in  the  appropriate  chapters  in  this 
book,  here  only  a  few  points  of  a  more  general  character  should  be 
mentioned. 

Broadly  speaking,  glands  are  organs  consisting  of  one  or  more  ele- 


GLANDS 


279 


EpitheimmlV:^si  W^^Lf 


Tunica 
propria 


1 1 


Submucosa 


Muscle  fibers 


Fig.  203. — Vertical  Section  through  the  Mucous  Membrane  of  the  Lip  of  an  adult  Man. 
X30.  (Stohr.)  1,  Papilla;  2,  excretory  duct;  the  lumen  is  cut  at  only  one  point;  3,  accessor}' 
gland;  4,  a  branch  of  the  excretory  duct  in  transverse  section;  5,  gland  bodies  grouped  into 
lobules  by  connective  tissue;  6,  a  gland  tubule  in  transverse  section. 


Part  of  an  Excretory  Duct  ^^&j^^^|S|l 


A  Crescent  Consisting 
of  Eight  Serous  Cells 


£*W* 


>. 


3>  -^F?|r^\n  J^W 


Tangential 
Section  of 
Serous  Cells 


Cross  Section 
with  Mucous 
Cells  and  (left) 
thick  Men- 
brana  Propria 


Lumen 


Fig.  204. — Section  of  a  Human  Sublingual  Gland.     X252.     (Stohr.) 


280 


HISTOLOGY 


mentary  tissues  but  in  which  the  epithelial  tissue  takes  the  leading  part 
in  performing  the  function  of  the  given  organ.  The  function  of  glands 
is  to  produce  various  substances  of  great  importance  to  the  economy  of 
the  organism;  and  the  epithelial  cells  in  the  glands  are  the  elements  which 
are  specialized  and  adapted  to  do  the  work  of  secreting  these  substances. 
In  lower  animals  we  find  distributed  among  other  epithelial  cells  indi- 
vidual secreting  cells  which  are  known  as  unicellular  glands.     In  higher 


Excretory  duct 


Traces  of  secre- 
tory ducts 


End  pieces 


Fig.  205.- — Diagram  of  the  Human  Sublingual  Gland.     {Stohr.) 


animals  we  also  find  in  various  localities  individual  cells  which  are 
endowed  with  the  faculty  of  secretion,  and  these  cells  are  known  as 
goblet  cells.  As  a  rule,  however,  glands  consist  of  groups  of  cells  united 
together  for  the  common  function  of  secretion.  The  secreting  cells  are 
arranged  in  the  form  of  tubes  or  sacs  and  the  substances  which  they  pro- 
duce are  conveyed  to  the  surface  through  outlets  known  as  ducts.  Owing 
to  the  fact  that  the  secreting  cells  in  glands  are  arranged  in  a  variety  of 
shapes,  the  following  classification  is  in  vogue: 

I.  Tubular  Glands. 

a.  Simple;  b.  compound. 
II.  Saccular  or  alveolar  Glands  or  racemous  glands, 
a.  Simple;  b.  compound. 

III.    TUBULO-ALVEOLAR   GLANDS. 

Glands  are  situated  either  within  the  surface  coverings  themselves  as 
for  example  in  the  mucous  membrane  of  the  digestive  tract,  or  they  are 


GLANDS 


28l 


imbedded  in  connective  tissue  beneath  the  mucous  membrane  known  as 
submucosa  as  for  example  in  the  skin  or  in  the  mucous  membrane  of  the 
lip.  A  number  of  glands  are  located  as  separate  bodies  imbedded 
among  other  important  organs,  as  for  example  the  various  salivary  glands, 
and  communicate  with  the  surface  upon  which  they  pour  their  products 
by  means  of  long  ducts.  A  description  of  the  various  glands  in  detail 
will  be  found  in  the  following  chapters. 

Beside  the  type  of  glands  just  described,  there  are  a  number  of  glands 
which  produce  substances  of  very  great  importance  in  the  economy  of 
the  body,  but  these  substances  are  not  conveyed  to  the  surface  and  there- 


x- 


> 


#.* 


-~  J>,    —Qfc  •*  V    *?  -   i 


mm 


.  > v: 


*■*  i 


n 


Fig.  206. — Longitudinal  Section  of  a  Lymph  Node.     (Radasch.) 
a,  Hilus;  b,  arteriole;  c,  venous  sinuses;  d,  adipose  tissue;  e,  secondary  nodule  of  cortex; 
/,  vein  of  medulla;  g,  subcapsular  lymph  sinus;  h,  germinal  center  of  secondary   nodule; 
i,  i,  trabecular;  k,  capsule;  /,  lymph  sinus;  m,  medullary  cord. 


fore  these  glands  have  no  direct  outlets  in  the  form  of  ducts  and  are  known 
as  ductless  or  internally  secreting  glands.  The  substances  produced  by  these 
glands  come  in  close  contact  with  the  blood  through  the  walls  of  the  capil- 
lary blood-vessels  distributed  throughout  the  texture  of  these  organs  and 
are  taken  into  the  blood  and  thus  become  distributed  throughout  the  body. 
The  glands  belonging  to  this  group  are  the  thyroid  bodies,  the  parathyroids, 
the  adrenal  and  the  hypophysis.  The  significance  of  these  organs  and 
their  products  is  still  a  subject  of  considerable  discussion. 

There  is  also  a  third  group  of  organs,  which  have  long  been  classi- 
fied as  glands  and  even  at  present  are  occasionally  considered  under 
that  heading,  the  so-called  lymph  glands.  It  was  stated  on  page  267 
that  the  so-called  lymphoid  cells  aggregate  in  larger  quantities  within 


282  HISTOLOGY 

the  meshes  of  areolar  tissue  and  form  the  lymphoid  tissue.  In  some 
localities  we  find  more  dense  aggregations  of  lymphoid  cells  which  are 
known  as  lymph  follicles.  In  the  center  of  the  follicle,  the  cells  are 
more  loosely  arranged  and  quite  a  number  of  mitotic  figures  can  be 
observed.  This  indicates  that  multiplication  of  cells  is  taking  place 
and  therefore  the  lymph  follicles  are  considered  as  the  main  localities 
where  lymph  cells  originate  in  the  adult.  In  various  regions  of  the 
body  are  found  interposed  in  the  lymphatic  circulation  a  number  of 
lymph  follicles  grouped  together  and  surrounded  by  a  connective  tissue 
capsule,  and  these  are  known  as  lymph  nodes  or  incorrectly  lymph 
glands.  As  examples  of  such  nodes  may  be  mentioned:  cervical  nodes, 
parotid  nodes,  lingual  nodes,  submaxillary  nodes,  etc.  If  we  recall, 
however,  that  under  the  name  gland  we  understand  an  epithelial  organ 
the  cells  of  which  are  endowed  with  the  power  of  secretion  it  is  obvious 
that,  when  this  name  is  applied  to  an  organ  consisting  of  lymphoid 
cells  which  never  secrete,  it  is  a  misnomer. 


CHAPTER  IV. 

The  Mucous  Membrane  of  the  Mouth;  of  the  Lips;  of  the  Cheeks; 
of  the  Gums ;  of  the  Roof  of  the  Mouth,  Hard  and  Soft  Palate ; 
of  the  Floor  of  the  Mouth;  the  Tongue. 

HISTOLOGY  OF  THE  TISSUES  OF  THE  MOUTH. 

Mucous  Membrane  of  the  Mouth. — The  mucous  membrane  lining 
the  cavity  of  the  mouth  consists  of  two  parts — the  epithelium  and  the 
tunica  porpria;  beneath  the  latter,  and  forming  the  deeper  part  of  the 
mucous  membrane,  is  the  submucosa. 


Older  layer 
of  Cells 


Infant  Lay- 
er of  Cells 


Connective 
Tissue 


Fig.  207. — Vertical  Section,  Mucous  Membrane  of  the  Mouth,  Human  Embrvo.      X150. 


The  epithelium  of  the  mouth  is  a  thick,  stratified,  squamous  epithe- 
lium, the  most  superficial  cells  being  scale-like  or  horn-like.  The  cells 
are  arranged  similar  to  those  in  the  epiderm,  the  lower  layers  are 
columnar  in  form,  and  contain  very  little  pigment. 

283 


284  HISTOLOGY 

The  tunica  propria  is  a  somewhat  dense  feltwork  of  interlacing  connect- 
ive-tissue bundles,  interspersed  with  elastic  fibers.  The  tunica  propiia 
penetrates  the  epithelium  in  the  form  of  cylindric  or  conic  papillae,  which 
differ  in  length  with  the  variation  in  the  thickness  of  the  epithelium. 
As  the  mucosa  is  usually  thickest  in  the  lips,  gums,  soft  palate,  and  uvula, 
accordingly  the  papillae  are  of  the  greatest  length  in  these  parts.  The 
tunica  propria  passes  into  the  submucosa  so  gradually  that  a  positive  line 
of  demarcation  cannot  be  established. 

The  submucosa  consists  of  a  bundle  of  fibrous  connective  tissue  with 
but  few  elastic  fibers.  This  structure  is  somewhat  loose  in  texture  and 
is  loosely  attached  to  the  underlying  periosteum.  Over  the  major  portion 
of  the  gums  and  the  entire  hard  palate  the  submucosa  is  attached  to  the 
bones  of  the  mouth  through  the  medium  of  their  periosteal  covering. 
It  is  in  this  loosely  constructed  tissue  that  the  glands  of  the  mucous  mem- 
brane are  situated.  These  are  for  the  most  part  branched,  tubular,  mucous 
glands.  Besides  adipose  tissue  in  the  form  of  groups  of  fat-cells,  striped 
muscular  tissue  is  present  in  the  submucosa.  In  some  parts  of  the  mouth 
this  tissue  forms  a  conspicuous  portion— namely,  in  the  sphincter  muscle 
of  the  lips  (orbicularis  oris) ;  also  in  the  soft  palate,  uvula,  and  pillars  of 
the  fauces. 

The  blood-supply  to  the  mucous  membrane  of  the  mouth  is  principally 
distributed  in  two  systems,  the  larger  vessels  to  the  submucosa  and  the 
capillaries  to  the  tunica  propria.  The  larger  vessels  break  up  and  send 
a  dense  network  of  capillaries  through  its  substance  and  to  the  numerous 
papillae  which  extend  into  the  epithelium.  Numerous  veins  ramify 
through  the  superficial  part  of  the  tunica  propria.  The  lymphatics  form 
two  networks,  the  submucosa  giving  place  to  the  coarser  vessels,  while 
the  fine  parts  are  distributed  to  the  tunica  propria. 

Nerve-supply  to  the  Mucous  Membrane  of  the  Mouth. — In  the  sub- 
mucosa the  medullated  nerve-fibers  form  a  wide-meshed  reticulum,  from 
which  numerous  primitive  fibrillae  pass  to  the  tunica  propria,  where  they 
terminate  or  continue  as  non-medullated  nerve-fibers,  and  penetrate  the 
papillae  of  the  epithelium,  forming  networks. 

Mucous  Membrane  of  the  Lips. — Beginning  as  a  direct  continuation 
of  the  integument  or  external  covering  of  the  lips,  the  labial  covering, 
including  the  integument,  may  be  divided  into  three  parts — namely,  a 
cutaneous  portion  (best  described  in  this  connection),  a  transitory  portion, 
and  a  mucomembranous  portion. 

The  cutaneous  portion,  covered  by  a  thin  epidermis,  consists  of  a 
double  layer  of  somewhat  flattened  epithelium.     Immediately  beneath 


MUCOUS    MEMBRANE    OF   THE    LIPS  285 

this  is  a  thin,  cellular,  mucous  layer,  the  cells  composing  it  being  spheroid 
in  form,  and  containing  nuclei  which  are  proportionately  large.  Sub- 
jacent to  this  is  the  cutis,  composed  of  fasciculi  of  fibers  intersecting 
and  closely  woven  together,  the  principal  fibers  passing  toward  the  free 
border  of  the  mucous  membrane  covering  the  contiguous  surface  of  the  lip. 
These  fibers  are  for  the  most  part  connective-tissue  fibers,  intermingled 
with  elastic-tissue  fibers.  Numerous  small,  vascular  papillae  are  found 
upon  the  surface  of  the  cutis;  these  are  cylindric  or  conic  in  form,  and 
project  for  some  distance  into  the  rete  mucosa — the  lower  layers  of 
living  cells  of  the  epidermis.  Equally  distributed  at  various  depths  in 
this  tissue  are  numerous  hair-  and  sebaceous  follicles.  The  general 
direction  of  the  hair-follicles  in  the  upper  lip  is  downward,  while  those 
occupying  the  lower  lip  are  turned  upward.  Other  than  the  distinction 
noted  by  the  difference  in  color  of  the  parts,  the  cutaneous  portion  may 
be  distinguished  from  the  transitional  mucous  membrane  by  the  absence 
of  hair-follicles  and  sebaceous  glands  in  the  latter. 

The  transitional  portion  of  the  mucous  membrane  of  the  lips  is  out- 
lined externally  by  the  outer  border  of  the  red  portion  of  the  lips,  and 
internally  by  that  prominent  part  of  the  labial  convexity  which  comes  in 
contact  with  the  opposing  labial  fold,  leaving  the  transitional  portion 
exposed  to  view  when  the  lips  are  in  occlusion.  The  epithelial  layer 
of  this  surface  does  not  begin  where  the  hair-follicles  cease  to  exist,  a 
slight  interspace  appearing  upon  the  cutaneous  portion  which  is  devoid 
of  these  follicles.  At  its  line  of  beginning  the  transitional  portion  of  the 
labial  mucous  membrane  is  quite  thin,  but  rapidly  increases  in  thickness 
in  passing  toward  the  mucomembranous  portion.  Superficially  the 
cells  are  much  flattened,  closely  associated  with  one  another,  and  devoid  of 
nuclei.  The  cells  of  the  middle  and  deeper  layers  are  oblong  or  spheric, 
and  provided  with  irregularly  shaped  nuclei.  The  chief  fibrous  tissues 
of  the  transitional  portion,  which  are  thinnest  at  the  point  where  the 
hair-follicles  cease  to  exist,  are  united  into  flexiform  fasciculi,  which  are 
separated  at  various  points  to  give  passage  to  numerous  minute  blood- 
vessels. The  fibrous  tissues  increase  in  thickness  as  the  mucomembran- 
ous portion  is  approached.  Numerous  thin  and  somewhat  elongated 
papillae  are  distributed  over  the  surface  of  the  transitional  portion. 

The  mucomembranous  portion  of  the  mucous  membrane  of  the  lips 
includes  all  that  portion  covering  the  labial  folds  within  the  mouth, 
beginning  at  the  line  of  occlusion  on  the  contiguous  surface  and  extend- 
ing to  the  gums.  The  epithelium  is  much  thicker  than  that  previously 
described   and  presents  the  characteristic  layers  common  to  stratified, 


286 


HISTOLOGY 


squamous  epithelium.  Superficially  the  cells  are  flattened  and  tubular, 
provided  with  nuclei  of  similar  form.  In  the  middle  layer  the  cells  are 
flattened  and  oblong,  followed  in  the  deeper  layer  by  irregulary  formed 
nucleated  cells.  A  variety  of  fibers  make  up  the  structure — one  class 
fine  in  texture  and  united  into  fasciculi,  intermingled  with  elastic  fibers, 
together  with  another  set  of  coarse,  strongly  looped  fibers.  Whenever 
the  fibers  of  the  tunica  propria  assume  a  definite  general  direction,  they 
are  horizontal,  passing  from  right  to  left  and  encircling  the  oral  aperture. 
The  tunica  propria  is  beset  with  numerous  conic  papillae  which  project 


Embryonal 

Mucous 
Membrane 


Buccal  Cavity 


Fig.  208. — Vertical  Transverse  Section  through  Head  of  Human  Embryo,  about  the  Sixth 
Week,  showing  Single  Buccal  Cavity.      X30. 


into  the  epithelium;  these  are  longest  where  the  epithelium  is  thickest. 
The  mucous  membrane  forming  the  labial  frena  is  covered  by  an  epithelial 
layer  which  is  much  thinner  than  that  distributed  to  other  parts  of  the  lips. 
The  fibers  in  these  are  irregularly  distributed,  and  the  papillae  are  small 
and  not  so  numerous.  The  coronary  arteries  and  their  accompanying 
veins  course  through  the  lips  near  the  junction  of  the  transitional  with 
the  mucomembranous  portion  of  the  mucous  membrane. 

Mucous  Membrane  of  the  Cheeks. — The  mucous  membrane  of 
the  cheeks  presents  but  little  variation  in  its  structure  from  that  of  the 
mucomembranous  portion  of  the  labial  mucous  membrane.  The  buccal 
epithelium  is  the  same  in  structure  and  thickness  as  that  of  the  lips, 


MUCOUS  MEMBRANE  OF  THE  GUMS 


287 


excepting  the  disposition  of  cells  in  the  middle  layer,  where  they  are  greater 
in  number  and  more  closely  associated,  being  somewhat  distorted  by  con- 
tact. The  papillae,  which  project  from  the  mucosa  into  the  epithelium, 
are  somewhat  broad  at  their  base,  with  elongated  extremities,  the  height 
of  which  is  quite  variable,  in  some  instances  penetrating  well  into  the 
epithelium,  at  others  merely  entering  its  deeper  layer.  At  the  anterior 
portion  of  the  cheek,  or  that  in  the  region  of  the  angle  of  the  mouth, 


Fig.  20Q. — Section    through    the  Mucous  Membrane  of  the  Cheek,  showing  the  Papillae  of 
the  Mucosa  in  Transverse  Section. 

the  mucous  membrane,  by  its  submucous  portion,  is  in  immediate  contact 
with  the  fibers  of  the  buccinator  muscle,  and  throughout  the  entire  surface 
of  the  cheek  it  is  closely  associated  with  this  muscle.  The  membrana 
propria  is  dense  immediately  beneath  the  epithelium,  but  as  the  buccinator 
is  approached  it  becomes  much  less  so. 

Mucous  Membrane  of  the  Gums. — The  mucous  membrane  cover- 
ing these  parts  is,  on  account  of  the  numerous  tendinous  fasciculi  which 
enter  into  its  construction,  extremely  dense  and  tough,  these  character- 
istics being  more  strongly  manifest  here  than  in  any  other  portion  of  the 


258  HISTOLOGY 

oral  mucous  membrane.  These  qualities  arc  especially  pronounced 
about  the  gingival  margins  and  over  the  major  portion  of  the 
alveolar  walls,  being  closely  bound  down  to  the  bone  by  direct  pro- 
longations of  the  tendinous  fasciculi  of  the  periosteum  which  penetrate 
the  membrane.  As  the  gingival  mucous  membrane  passes  into  that  of 
the  lips  and  cheeks  it  gradually  becomes  less  dense.  The  epithelium 
of  the  mucous  membrane  of  the  gums  is  composed  of  lamina  of  tessellated 


Epithelium 


Papilla 


Fig.  210. — Section  through  the  Gums,  showing  Epithelium  and  Basement  Membrane. 


and  ribbed  cells.  The  superficial  cells  are  the  flattened  cells  of  pave- 
ment epithelium;  subjacent  to  this  they  become  thicker  and  deeply  ribbed, 
while  the  deepest  cells  are  conic  or  cylindric  with  conic  extremities.  The 
tissue  composing  the  tunica  propria  is  made  up  of  flattened  fasciculi  of 
connective  tissue,  the  fibers  of  which  run  parallel  with  one  another. 
Numerous  elastic  fibers  are  also  present.  Three  sets  of  fibers  are  to  be 
distinguished  in  the  mucous  membrane  of  the  gums — those  which  run 
vertically,  those  which  pass  in  a  horizontal  direction,  and  those  which 
radiate  or  are  distributed  fan-like.     Of  the  first-named,  the  fibers  extend 


MUCOUS  MEMBRANE  OF  THE  ROOF  OF  THE  MOUTH       289 

from  above  downward;  in  the  second  class  they  pass  from  right  to  left 
parallel  with  the  surface;  the  third  class,  including  those  fibers  which  are 
reflected  from  the  alveolodental  membrane,  are  distributed  in  fasciculi 
about  the  margins  of  the  alveoli. 

Mucous  Membrane  of  the  Roof  of  the  Mouth.  Hard  Palate. — 
The  mucous  membrane  covering  the  hard  palate  is,  in  very  many  respects, 
dissimilar  to  that  surrounding  the  necks  of  the  teeth  and  forming  the 
palatogingival  margins.  Like  the  mucous  membrane  of  the  gums,  that 
overlying  the  hard  palate  is  dense  and  tough.  The  papillae  of  the  tunica 
propria,  which  penetrate  the  epithelium,  are  not  so  numerous  as  those 
upon  the  gums.  In  the  posterior  third  of  the  hard  palate  they  are  some- 
what more  numerous  and  generally  a  little  more  prominent  than  those  in 
the  anterior  portion.  In  the  median  raphe  and  over  the  rugae,  the  papillae 
are  especially  sparingly  distributed.  The  epithelium  is  of  the  pavement 
variety,  somewhat  thinner  in  front  than  behind,  the  cells  being  more 
freely  distributed  at  some  points  than  at  others.  The  mucous  membrane 
of  the  hard  palate  is  less  in  thickness  anteriorly  than  posteriorly.  The 
distribution  of  the  fibers  is  such  that  they  radiate  from  the  alveolar  borders 
toward  the  center  of  the  palate,  the  anterior  fibers  passing  obliquely 
backward,  while  those  from  the  lateral  walls  pass  parallel  with  one 
another  to  the  median  line.  For  the  most  part  the  fibers  are  broad  and 
form  a  plexus  between  the  epithelium  and  the  submucous  tissue.  The 
submucous  tissue  is  sparingly  distributed  over  the  central  portion  of  the 
hard  palate,  but  laterally  is  somewhat  more  abundant,  containing  a  few 
fat-cells. 

Soft  Palate,  Uvula,  and  Fauces.— Passing  backward  from  the  posterior 
margin  of  the  hard  palate,  the  mucous  membrane  overlies  the  fibrous 
aponeurosis  of  the  soft  palate  and  its  median  and  lateral  prolongations — 
the  uvula  and  pillars  of  the  fauces.  The  epithelium  is  of  the  laminated 
pavement  variety,  with  the  deeper  cells  larger  than  those  placed  super- 
ficially. The  substance  of  the  mucous  membrane  is  composed  of  fasciculi 
of  connective  tissue,  intermingled  with  a  plexus  of  elastic  fibers.  The 
fibers  are  distributed  in  three  principal  directions — from  side  to  side, 
or  horizontally,  longitudinally,  and  obliquely.  The  oblique  fibers  are 
instrumental  in  forming  the  submucous  tissue  of  both  the  soft  palate  and 
uvula.  Numerous  conic  papillae  project  from  the  tunica  propria  into  the 
epithelium;  these  are  larger  and  more  numerous  on  the  uvula  than  on  the 
soft  palate.  The  tunica  propria  is  somewhat  variable  in  thickness  to 
accommodate  the  glands,  which  are  more  or  less  numerous,  and  present 
in  greater  numbers  in  one  instance  than  in  another.  In  general  the 
19 


290 


HISTOLOGY 


membrane  as  a  whole  is  thinnest  along  the  margin  of  the  hard  palate, 
gradually  increasing  in  thickness  as  the  free  border  is  approached.  The 
folds  of  mucous  membrane  forming  the  pillars  of  the  fauces  present  no 
peculiarity  differing  from  that  of  the  soft  palate,  save  a  more  generous 
supply  of  elastic  fibers. 

Mucous  Membrane  of  the  Floor  of  the  Mouth. 

The  Tongue. — The  entire  unattached  surface  of  the  tongue  is  covered 
by  a  reflection  of  the  mucous  membrane  of  the  floor  of  the  mouth.  In  this 
organ  the  general  structure  of  the  mucous  membrane  does  not  vary  from 
that  of  other  oral  mucous  membrane,  being  composed  of  an  epithelium, 
a  tunica  propria,  and  a  submucosa.     The  mucous  membrane  covering 


Epithelium 


Fig.  211. — Longitudinal  Section  through  Mucous  Membrane  of  the  Human  Tongue.  X  20. 

the  dorsum  of  the  tongue  presents  special  characteristics,  which  differ 
from  that  of  the  under  surface  and  the  floor  of  the  mouth  in  general. 
In  the  former  location  the  papillary  elevations  of  the  tunica  propria  are 
conspicuously  developed,  and  with  their  covering  of  stratified,  scaly 
epithelium  cause  the  peculiar  furred  appearance.  Three  classes  of 
papillae  are  distinguished,  named,  in  accordance  with  their  form,  filiform 
papillae,  fungiform  papillae,  and  circumvallate  papillae. 

The  filiform  papilla,  which  are  very  numerous  over  the  entire  dorsum 
and  sides  of  the  tongue,  are  conic  and  frequently  prolonged  into  numerous 
horn-like  processes,  known  as  secondary  papillae.  As  elevations  from 
the  tunica  propria  they  are  composed  of  well-defined  fibrillated  tissue, 


MUCOUS    MEMBRANE    OF    THE    FLOOR    OF    THE    MOUTH 


>QI 


intermingled  with  numerous  elastic  fibers.  The  pavement  epithelial  cells 
are  found  overlapping  one  another,  and  provided  with  processes  which 
project  beyond  the  papillae. 

The  fungiform  papilla  are  also  distributed  over  the  entire  dorsum 
and  sides  of  the  tongue,  but  are  somewhat  less  numerous  than  the  filiform 
variety.  They  appear  as  well-defined  elevations,  and  are  connected 
with  the  tunica  propria  by  a  constricted  portion  or  neck.  The  entire 
free  or  rounded  surface  of  these  papillae  is  beset  with  secondary  papillae. 
The  epithelium  is  slightly  thinner  than  that  over  the  filiform  papillae,  this 
being  the  principal  distinguishing  feature.  The  numerous  capillaries 
produce  a  rich  red   color,  plainly  observable  through  the  transparent 


Fig.  212. 


-Longitudinal  Section  of  the  Mucous  Membrane  of  the  Human  Tongue,  showing 
the  Fungiform  and  Secondary  Papillae.      X  80. 


epithelium.     Connective-tissue    bundles    make    up    the    bulk    of    these 
papillae,  few  elastic  fibers  being  present. 

The  circumvallate  papilla  are  placed  on  the  posterior  portion  of  the 
dorsum  of  the  tongue,  and  are  few  in  number  (eight  to  sixteen).  They  are 
much  larger  than  those  already  described,  and  in  general  resemble  modified 
fungiform  papillae.  They  are  flattened  and  broad,  and  differ  from  the 
fungiform  by  having  a  circular  furrow  or  wall  surrounding  them.  Second- 
ary papillae  are  present  on  the  free  surface  only,  the  sides,  and  in  some 
instances  the  walls  surrounding  them,  being  occupied  by  the  end  organs 
of  the  special  sense  of  taste — the  taste-buds.  Other  taste-buds  are  found 
upon  the  lateral  margins  of  the  tongue  posteriorly,  nestled  in  a  group  of 


292 


HISTOLOGY 


parallel  folds  of  mucous  membrane — the  papillae  foliata.  The  connective 
tissue  within  these  papillae  is  similar  to  that  in  the  fungiform  papillae. 
On  other  parts  of  the  tongue,  or  those  portions  not  occupied  by  these 
specially  constructed  papillae,  the  epithelium  is  similar  to  that  in  other 
parts  of  the  mouth.  The  tunica  propria  is  less  in  thickness  in  and  about 
the  tip  of  the  tongue,  and  is  intimately  connected  with  the  subjacent 
muscular  structure.  As  the  root  of  the  organ  is  approached,  the  tunica 
propria  becomes  thicker  and  more  dense.     The  submucosa  is  especially 


Fig.  213. — Section  through  Epithelium,  Near  the  Tip  of  the  Tongue.      X40. 

intimately  connected  with  the  underlying  parts  at  the  margins  and  tip 
of  the  tongue.  The  extreme  portion  of  the  root  of  the  tongue  has  its 
mucous  membrane  particularly  modified  by  a  special  aggregation  of 
adenoid  tissue — developed  lymph-nodules.  These  are  large  and  readily 
perceptible  to  the  naked  eye.  They  are  provided  with  a  central  opening, 
which  dips  down  into  a  well-defined  vault  or  crypt,  which  is  lined  by  a 
reflection  of  the  stratified  oral  epithelium. 

Blood-supply  to  the  Mucous  Membrane  of  the  Mouth. — The  oral 
mucous  membrane  derives  its  supply  of  blood  from  numerous  branches 
of  the  external  carotid  artery — namely,  the  superior  and  inferior  coro- 
nary, buccal,  lingual,  transverse  facial,  pterygopalatine,  and  the  alveolar. 
Entering  the  submucosa,  the  minute  terminal  branches  of  these  arteries 
are  distributed  parallel  to  the  surface,  and  by  anastomosis  form  plexuses 
from  which  other  minute  branches  are  given  off  to  supply  the  papillae  of 
the  tunica  propria.  After  coursing  through  the  papillae  the  blood  is 
discharged  into  a  similar  venous  plexus,  and  thus  conveyed  from  the 


BLOOD-SUPPLY    TO    THE    MUCOUS    MEMBRANE    OF    THE    MOUTH    293 

parts.  In  a  like  manner  the  mucous  membrane  and  papillae  of  the 
tongue  are  supplied,  branches  of  the  lingual  artery  conveying  the  blood 
to  the  parts.  The  dorsalis  linguae  supplies  the  mucous  membrane  of  the 
dorsum  of  the  tongue  and  pillars  of  the  fauces,  while  the  ranine  artery 
by  its  minute  branches  supplies  the  remaining  mucous  membrane.  Each 
papilla  is  entered  by  two  or  more  arterial  terminals,  which  divide,  anas- 
tomose, and  finally  send  off  capillary  branches  to  the  secondary  papillae. 
Nerve-supply  to  the  Mucous  Membrane  of  the  Mouth. — The 
distribution  of  the  nerve-fibers  to  the  oral  mucous  membrane  is  approxi- 
mately similar  in  all  parts.  The  fibers,  which  are  of  the  medullated 
variety,  are  first  distributed  to  the  submucosa,  forming  a  wide-meshed 
reticulum.  From  this  fibers  are  given  off  to  the  tunica  propria,  terminat- 
ing in  end-bulbs,  or,  after  losing  their  medullary  sheath,  are  distributed 
to  the  epithelium,  where  their  free  extremities  lie  between  the  epithelial 
cells.  The  nerves  of  the  mucous  membrane  of  the  tongue  (the  glosso- 
pharyngeal and  lingual  branch  of  the  fifth)  may  have  their  endings  similar 
to  those  in  other  parts  of  the  mouth,  or  they  may  be  intimately  associated 
with  the  taste-buds. 

Literature. 

Legros  and  Magitot,  1880. 

Klein,  "Structure  of  the  Oral  Lips,"  1868. 

Sebastian,  "Anatomy  and  Physiology  of  the  Labial  Glands,"  1842. 

Kolliker,  "  Mikroskopische  Anatomic" 

Kirke,  "Physiology.  " 

Strieker,  "Human  and  Comparative  Histology." 

Stohr,  "Text-book  of  Histology,"  1896. 


CHAPTER  V. 

Other  Structures   Within  the  Mouth. — The   Gums. — The  Mucous 

Membrane. — The  Alveolodental  Membrane. — 

Glands,  Ducts,  Etc. 

The  Gums  (Gingiva). — The  gums  are  formed  by  a  layer  of  tough 
fibrous  vascular  tissue,  covering  the  alveoli,  closely  attached  to  their 
periosteum,  and  provided  with  a  free  margin  (gingival  margin),  which 


Epithelium 


Alveolus 


Fig.  214. — Section  through  Gingivae  and  Alveolus. 

is  closely  molded  to  the  necks  of  the  teeth.  They  are  covered  on  both 
aspects  by  the  general  mucous  membrane  of  the  mouth,  that  overlying 
the  labial  and  buccal  surfaces  being  reflected  from  the  lips  and  cheeks, 

294 


THE    GUMS  295 

the  palatal  surface  being  continuous  with  that  of  the  hard  palate,  and 
the  lingual  surface  reflected  from  the  under  surface  of  the  tongue  and 
floor  of  the  mouth.  In  the  immediate  region  of  the  necks  of  the  teeth  the 
gums  are  especially  thin  and  hard,  being  closely  adherent  to  the  peri- 
osteum and  alveolodental  membrane  in  this  region.  In  passing  toward 
the  base  of  the  alveolar  walls  the  tissue  becomes  less  firmly  attached  to 
the  underlying  structure,  and,  when  finally  passing  into  the  mucous 
membrane  of  the  cheeks  and  lips,  is  quite  loose  and  flabby.  This  condi- 
tion also  prevails  on  the  lingual  aspect,  but  the  palatal  surface  remains 
firm  throughout,  the  entire  mucous  membrane  overlying  the  roof  of  the 
mouth  being  similar  in  structure  and  attachment  to  that  portion  imme- 
diately surrounding  the  necks  of  the  teeth.  In  various  situations  about 
the  labial,  buccal,  and  lingual  surfaces  of  the  gums  small  slender  folds 
of  mucous  membrane  are, found  extending  to  the  surrounding  tissues. 
These  folds,  which  act  as  a  bridle  or  curb  to  the  adjacent  movable  parts, 
are  known  as  the  frena  of  the  mouth.  The  principal  frena  are  found  at 
the  median  line,  and  are  three  in  number — the  frenum  labium  superioris, 
frenum  labium  inferioris,  and  the  frenum  linguae.  The  two  former 
extend  from  the  inner  surface  of  the  lips  to  the  gums,  to  which  their  extent 
of  attachment  is  somewhat  variable.  The  frenum  labium  superioris  is 
usually  much  larger  than  the  frenum  labium  inferioris,  and  its  attachment 
to  the  gum  frequently  extends  almost  to  the  gingival  border.  The  frenum 
lingua?  extends  from  the  under  surface  of  the  tip  of  the  tongue  to  the  lin- 
gual surface  of  the  inferior  gums.  This  is  a  much  stronger  fold  than 
those  connected  with  the  lips.  Similar  bridles  are  found  in  the  buccal 
region,  usually  near  the  bicuspid  teeth,  but  they  are  much  smaller  than 
those  at  the  median  line  The  gingival  margins,  or  that  portion  of  the 
gums  embracing  the  necks  of  the  teeth,  present  much  variety  in  outline. 
Instead  of  encircling  the  neck  of  the  tooth  in  a  direct  line,  the  margins 
are  made  up  of  a  series  of  semicircles.  Using  the  incisive  region  for 
reference,  the  labial  and  palatal  margins  are  concave  rootward,  while  the 
interproximate  spaces  are  partly  or  completely  filled  by  gum  tissue, 
having  the  outline  reversed  or  convex  in  the  direction  of  the  crowns  of 
the  teeth.  The  gingival  margin  is  also  termed  the  "free  margin  of  the 
gum,"  this  name  better  describing  its  extent.  As  previously  stated,  the 
gums  are  attached  to  the  periosteum  and  peridental  membrane,  but  in 
most  instances,  and  particularly  before  the  adult  period,  the  free  margins 
of  the  gums  extend  beyond  the  alveolodental  membrane,  the  limit  of  which 
is  formed  by  the  cervical  line.  That  portion  of  the  gum  margin  beyond 
the  cervical  line  is  in  close  contact  with  the  neck  of  the  tooth,  but  is  not 


296  HISTOLOGY 

adherent  to  it,  the  connecting  medium,  the  alveolodental  membrane,  not 
being  present  to  form  the  attachment.  The  curvature  of  the  gingival 
margins,  and  the  nature  of  the  tissues  which  enter  into  their  construction, 
are  usually  considered  as  strongly  indicative  of  the  temperament  of  the 
individual.  Thus,  in  the  bilious  temperament  the  margins  are  inclined 
to  angularity  and  the  tissues  rather  thick  and  firm.  In  the  sanguine  type 
the  outline  formed  is  almost  a  perfect  semicircle,  and  the  tissues  are  of 
moderate  thickness  and  firmness.  In  the  nervous  type  the  curvature 
strongly  parabolic,  and  the  tissues  firm  and  delicate.  In  the  lymphatic 
the  tissues  are  loose  and  thick,  and  the  curvature  is  long  and  poorly 
defined.  In  some  instances  the  interproximate  spaces  are  completely 
filled  by  the  gingivae;  in  others  the  space  is  only  partly  occupied  by  these 
tissues.  The  former  condition  is  present  when  the  proximate  surfaces  of 
the  teeth  are  nearly  or  quite  parallel  with  each  other,  thus  reducing  the 
capacity  of  the  space.  The  latter  condition  is  present  when  the  crowns 
of  the  teeth  are  bell-shaped  and  the  interproximate  spaces  extensive.  The 
labial  and  buccal  surfaces  of  the  gums  are  more  or  less  broken  by  numer- 
ous prominences  and  depressions,  all  of  which  accord  with  the  variations 
upon  the  surface  of  the  bone  beneath. 

Some  distinction  should  be  made  between  the  gingival  line  and  the 
cervical  line,  the  former  referring  to  the  free  margin  of  the  gum,  and  not  a 
fixed  line,  while  the  latter  refers  to  that  positive  line  established  on  the 
tooth  by  the  union  of  the  enamel  and  cementum. 

Mucous  Membrane  of  the  Mouth. — The  term  "membrane"  in  a 
general  sense  is  one  applied  to  thin  layers  of  tissue,  somewhat  elastic 
and  of  a  whitish  or  reddish  color.  Such  tissues  are  found  lining  either 
closed  cavities  or  canals  which  open  externally,  absorbing  or  secreting 
fluids,  and  enveloping  various  organs.  The  simple  membranes  are  of 
three  varieties,  being  either  mucous,  serous,  or  fibrous.  The  mucous 
membranes  are  so  called  from  the  clear  viscid  fluid  (mucus)  which  they 
secrete.  They  line  the  various  cavities  or  tracts  of  the  body  which  com- 
municate with  the  exterior.  The  three  grand  divisions  of  mucous  mem- 
brane are  those  lining  the  digestive,  respiratory,  and  genito-urinary  pas- 
sages. Lining  the  entire  cavity  of  the  mouth  we  find  the  beginning  of  the 
digestive  tract,  being  continuous  with,  in  many  respects  similar  to,  the  skin 
on  the  exterior  and  performing  similar  functions  within.  It  is  soft,  smooth, 
and  velvety,  of  a  bright  red  color,  and  quite  vascular;  it  is  covered  on  the 
exterior  by  a  layer  of  epithelial  cells  overlying  the  vascular  parts.  Imme- 
diately beneath  this  is  a  network  of  fibrous  connective  tissue  forming  the 
proper  mucous  membrane,  and  still  deeper  is  a  third  layer,  somewhat 


THE   ALVEOLODENTAL    PERIOSTEUM    OR    ROOT    MEMBRANE  297 

loose  in  texture,  but  composed  of  fibrous  connective  tissue,  the  submucous 
membrane.  The  oral  mucous  membrane,  at  its  point  of  beginning  on  the 
contiguous  surface  of  the  lips,  is  endowed  with  keen  sensibility;  it  is  dry, 
bright  red  in  color,  and  plentifully  supplied  with  vascular  papillae,  in 
many  of  which  are  sensory  nerve  terminals.  Distributed  along  the  line 
of  junction  between  the  integument  and  the  mucous  membrane  are 
numerous  sebaceous  follicles,  which,  however,  are  devoid  of  hair-bulbs. 
The  characteristic  dryness  of  this  surface  gradually  becomes  changed  to  a 
mucus-secreting  one,  as  that  part  of  the  membrane  lining  the  interior  of 
the  lips  is  approached.  Distributed  over  the  surface  of  the  labial  mucous 
membrane  are  a  number  of  minute  openings,  the  mouths  of  the  labial 
glands,  which  lie  immediately  beneath  the  membrane.  The  buccal  mu- 
cous membrane,  or  that  lining  the  cheeks,  is  similar  to  that  covering  the 
internal  surface  of  the  lips.  It  is  penetrated  at  various  points  by  the 
mouths  of  the  buccal  glands,  which,  in  general,  are  smaller  and  less 
numerous  than  the  labial  glands.  In  the  region  of  the  second  molar 
teeth  the  membrane  is  broken  by  four  or  five  openings  of  larger  size, 
which  communicate  with  the  molar  glands.  The  mucous  membrane 
covering  the  hard  palate  is  thick  and  firm,  less  brilliant  in  color  than  that 
covering  the  cheeks  and  lips,  and  firmly  bound  down  to  the  periosteum. 
Running  from  before  backward  at  the  median  line,  the  membrane  is 
formed  into  a  slight  fold,  the  median  raphe,  while  near  the  anterior  portion 
of  the  palate  are  a  number  of  fantastically  arranged  folds,  the  ruga  (see 
General  Description  of  the  Mouth).  The  thin  but  rather  dense  fibrous 
aponeurosis  forming  the  soft  palate  is  covered  anteriorly  by  the  oral  mu- 
cous membrane.  Suspended  from  the  center  of  the  free  margin  of  the  soft 
palate  is  the  uvula,  which  is  likewise  covered  by  mucous  membrane,  and 
from  the  base  of  this,  on  either  side,  are  two  muscular  folds,  which  extend 
outward  and  downward,  forming  the  anterior  and  posterior  pillars  of  the 
fauces.  The  mucous  membrane  covering  the  tongue  has  already  been 
described  in  connection  with  that  organ.  From  the  mouth  the  digestive 
mucous  membrane  passes  through  the  fauces,  pharynx,  and  esophagus 
to  the  stomach,  and  is  so  continued  throughout  the  whole  digestive 
tract.  Other  prolongations  also  pass  into  the  ducts  of  the  salivary 
glands. 

The  Alveolodental  or  Peridental  Membrane. 

This  membrane  invests  the  roots  of  the  teeth,  and  at  the  same  time 
lines  the  wall  of  the  alveoli.     Being  reflected  from  the  periosteum  cover- 

*  For  a  minute  description  of  the  mucous  membrane  of  the  mouth  see  Part  II. 


298 


HISTOLOGY 


ing  the  outer  alveolar  walls,  it  enters  the  alveolar  sockets  as  a  single 
membrane,  affording  nourishment  to  the  bone  on  one  side  and  to  the 
cementum  of  the  tooth  on  the  other.  It  is  a  connective  tissue  of  moderate 
density,  and  is  rich  in  its  nerve-  and  blood-supply.  The  general  direction 
of  its  fibers  is  transverse,  being  attached  at  one  extremity  to  the  alveolar 

wall    and    at   the    other    to    the 
***  cementum    of    the    root.     The 

connective-tissue  fibers  are  not 
merely  attached  to  the  surface 
of  the  calcified  structure,  but  the 
strength  of  this  attachment  is 
greatly  increased  by  the  passage 
of  the  fibers  into  the  substance  of 
the  bone  at  one  extremity  and 
into  the  lamella?  of  the  cementum 
at  the  other.  In  general,  the 
membrane  is  more  closely  ad- 
herent to  the  cementum  than  to 
the  bone,  usually  clinging  to  the 
former  when  removed  from  its 
socket.  The  nature  of  the  ar- 
ticulation between  the  tooth-root 
and  the  alveolar  socket,  to  the 
production  of  which  this  mem- 
brane so  largely  contributes,  is 
one  peculiar  to  itself.  While 
there  is  no  marked  mobility, 
there  is,  nevertheless,  sufficient 
elasticity  in  the  intervening  mem- 
brane to  provide  against  the 
severe  concussions  and  lateral 
strains  incident  to  mastication,  the  former  being  provided  for  by  the 
general  elasticity,  while  the  latter  is  cared  for  by  specially  distributed  fibers, 
which  serve  to  return  the  tooth  to  its  normal  position  when  slightly  rotated 
or  laterally  displaced.  This  elasticity  is  greatest  in  youth  and  up  to  the 
meridian  of  life,  after  which  time  it  gradually  becomes  less  pronounced. 
The  membrane  is  thickest  about  the  apical  ends  of  the  roots  and  in  the 
cervical  region,  and  the  distribution  of  the  fibers  at  these  points  is  some- 
what different  from  those  about  the  body  of  the  root.  In  the  former 
location  they  are  spread  out  fan-like  from  the  apex  of  the  root  to  attach 


Fig.  215. — Root  and  Membrane  of  Tooth. 
p,  p,  Peridental  membrane;  ap,  apical 
space;  a,  artery;  al,  al,  alveolar  process 
/,  /,  dental  ligament. 


THE  NERVE-SUPPLY    TO    THE    ALVEOLODENTAL    MEMBRANE  299 

themselves  to  the  surrounding  alveolar  wall,  while  in  the  latter  they 
pass  longitudinally  over  the  alveolar  margins  to  unite  with  the  periosteum 
of  the  parts.  In  conjunction  with  the  functions  already  mentioned,  the 
peridental  membrane  is  the  median  by  which  all  forces  applied  to  the 
tooth-surface  are  taken  up  and  conveyed  to  the  brain,  making  it  the  organ 
of  the  sense  of  touch  to  the  tooth.  In  a  normal  tooth  sensations  of  pain 
alone  are  conveyed  through  the  nerves  of  the  pulp.  The  nerves  of  the 
membrane  act  in  precisely  the  same  manner  as  do  other  sensory-nerve 
terminals,  being  influenced  by  the  slightest  touch  applied  to  the  surface 
of  the  tooth-crown.*  In  certain  conditions  of  defective  hearing  the  alveo- 
lodental  membrane  may  be  made  to  assist  this  function  by  the  use  of  an 
instrument  made  for  the  purpose  known  as  a  dentiphone. 

Blood-supply  to  the  Alveolodental  Membrane. — A  very  clear 
idea  of  the  blood-supply  to  this  membrane  may  be  obtained  from 
figure  215. 

After  entering  the  alveolar  socket  by  one  or  more  arterial  branches, 
the  thickest  portion  of  the  membrane  is  gained  where  a  number  of  smaller 
twigs  are  given  off,  one  or  more  of  which  enter  the  pulp-canal  of  the  tooth- 
root  through  the  apical  foramina  supplying  the  pulp,  which  in  turn 
supplies  the  tooth-structure  within,  while  the  others  ramify  through  the 
substance  of  the  alveolodental  membrane,  through  its  capillaries,  supply- 
ing the  cementum  from  without;  while  passing  through  the  membrane 
further  minute  branches  are  given  off  which  penetrate  the  walls  of  the 
alveolus  and  anastomose  with  the  arteries  which  supply  the  oral  mucous 
membrane,  in  this  manner  providing  a  generous  blood-supply  to  the  parts. 
Further  on  we  shall  see  that  the  tooth-pulp  and  the  alveolodental  mem- 
brane spring  from  the  same  source  (see  Development  of  the  Teeth),  and 
the  blood-supply  to  the  parts  during  the  saccular  stage  of  development 
is  alike  distributed  to  the  base  of  the  pulp  and  to  the  follicular  walls. 
After  the  completion  of  the  developmental  process  this  distribution  is  but 
little  changed,  the  blood-vessels  accommodating  themselves  to  the  altera- 
tions incident  to  the  generation  of  the  parts. 

The  Nerve-supply  to  the  Alveolodental  Membrane. — The  nerve- 
supply  to  this  membrane  is  distributed  in  a  manner  similar  to  the  blood- 
supply,  being  derived  from  filaments  given  off  from  the  dental  nerve  and 
entering  the  tooth-socket  by  the  side  of  the  blood-vessels,  and  by  numer- 
ous filaments  which  reach  the  structure  by  passing  through  the  many 
minute  canals  in  the  substance  of  the  alveolar  walls  and  the  intervening 
septa. 


300  HISTOLOGY 

GLANDS  OF  THE  MOUTH. 

The  glands  of  the  mouth  are  of  two  kinds,  being  either  serous  or 
mucous,  and,  as  they  differ  in  the  character  of  their  secretions,  so  they 
differ  in  structure.  The  mucous  glands  are  the  most  numerous,  and  are 
found  beneath  the  mucous  membrane  of  the  lips,  in  the  same  membrane 
lining  the  cheeks,  the  hard  and  soft  palate,  the  tonsils,  and  at  the  back  of 
the  tongue.  These  glands  are  quite  variable  in  size,  but  are  all  of  macro- 
scopic proportions,  appearing  when  examined  in  this  manner  as  minute 
whitish  bodies.  The  secretions  from  the  glands  are  poured  into  the 
mouth  through  small  ducts  which  pass  in  various  directions  through  the 
substance  of  the  mucous  membrane.  Beginning  as  a  single  duct  for  each 
gland,  they  pass  to  the  submucous  tissue,  here  branching  into  two  or 
more  smaller  ducts  terminating  in  alveoli,  the  number  and  size  of  these 
depending  upon  the  size  of  the  gland  with  which  they  are  connected. 
The  glands  are  variously  named  according  to  their  location,  those  occupy- 
ing the  lips  being  known  as  labial  glands;  those  of  the  cheeks,  the  buccal 
glands;  those  of  the  palate,  the  palatal  glands,  etc.  The  mucous  glands, 
although  differing  in  size,  are  similar  when  histologically  considered. 

The  Labial  Glands. — These  are  among  the  largest  mucous  glands 
of  the  mouth,  and  are  more  numerous  in  the  upper  than  in  the  lower  lip. 
The  form,  size,  and  location  of  the  labial  glands  may  best  be  studied 
by  dissection,  which  may  readily  be  accomplished  by  first  removing 
the  integument  and  muscular  tissues  from  the  parts,  when  they  will  be 
brought  into  view.  The  glands  are  irregularly  arranged,  and  are  most 
numerous  near  the  median  line.  The  body  of  each  gland  is  surrounded 
and  held  in  position  by  connective  tissue,  as  well  as  by  the  duct  connect- 
ing it  with  the  interior  of  the  mouth. 

Besides  the  mucous  glands  of  the  lips,  there  are  present  numerous 
sebaceous  glands.  These  are  somewhat  smaller,  and  are  situated  beneath 
the  mucous  membrane  covering  the  contiguous  surfaces  of  the  lips,  the 
numerous  ducts  leading  from  them  opening  upon  these  parts. 

The  Buccal  Glands. — The  glands  of  the  cheek,  otherwise  known 
as  buccal  glands,  are  similar  to,  but  smaller  than  those  of  the  lips,  and  are 
placed  between  the  submucous  tissue  and  the  buccinator  muscle.  These 
glands  also  pour  their  secretions  into  the  mouth  through  numerous  ducts 
which  pass  through  the  buccal  mucotis  membrane. 

In  the  region  of  the  third  molar  teeth  another  set  of  mucous  glands 
open  into  the  mouth,  known  as  the  molar  glands.  They  are  placed  between 
the  buccinator  and  masseter  muscles,  are  similar  in  construction,  and 


GLANDS    OF    THE    MOUTH  3OI 

secrete  a  like  fluid  to  those  previously  described,  being  larger  than  the 
buccal  and  smaller  than  the  labial  glands. 

Palatal  Glands. — Situated  between  the  mucous  membrane  of  the 
hard  palate  and  the  periosteum  are  numerous  mucous  glands  similar  to 
those  previously  described.  Provision  is  made  for  the  accommodation 
of  these  glands  by  many  small  depressions  in  the  bony  plates  (see  Fig.  6). 
They  are  irregularly  distributed  over  the  surface  of  each  lateral  half  of 
the  hard  palate,  but  are  absent  at  the  median  raphe  and  immediately 
beneath  the  rugae.  The  mucous  membrane  of  the  hard  palate  is  tense 
and  hard,  in  consequence  of  which  it  is  not  so  thick  as  the  buccal  and 
lingual  membranes,  and  the  ducts  from  the  numerous  glands  are  there- 
fore not  so  long.  The  glands  of  the  soft  palate,  uvulae,  and  fauces  are 
situated  beneath  the  deep  layer  of  mucous  membrane  covering  these 
parts,  in  the  former  structure  opening  on  both  the  oral  and  nasal  surfaces. 

Lingual  Glands. — The  glands  of  the  tongue  are  of  two  kinds — 
mucous  and  serous.  The  former  are  chiefly  found  at  the  back  part  of  the 
tongue,  but  a  few  of  smaller  size  are  present  near  the  tip.  The  serous 
variety  are  to  be  found  only  at  the  back  of  the  organ,  and  are  closely  asso- 
ciated with  the  taste  organs  in  this  region.  These  glands  are  assisted  in 
performing  their  function  by  being  placed  between  bundles  of  striped  mus- 
cular tissue,  the  activity  of  which  forces  the  secretions  to  the  surface 
by  compressing  the  glands.  While  the  majority  of  the  lingual  glands  are 
present  in  the  circumvallate  region,  a  number  are  found  distributed 
beneath  the  mucous  membrane  of  the  borders  and  tip  of  the  organ. 

The  Salivary  Glands  (Fig.  216). — These  glands,  while  outside  the 
mouth,  are  so  closely  associated  with  its  functions  that  a  brief  description 
will  be  presented.  The  chief  salivary  glands  are  six  in  number,  three  on 
each  side.  They  are  named  parotid,  submaxillary,  and  sublingual;  the 
former,  secreting  true,  thin,  watery  saliva  is  a  true  salivary  gland,  while 
the  latter  two  are  known  as  mixed,  or  mucosalivary  glands,  secreting 
both  mucus  and  saliva. 

The  Parotid  Gland. — This  gland  is  the  largest  of  the  three,  and  is 
placed  a  little  below  and  in  front  of  the  ear,  having  the  following  bounda- 
ries: Anteriorly,  by  the  ramus  of  the  mandible;  posteriorly,  by  the  styloid 
and  mastoid  processes  of  the  temporal  bone;  above,  by  the  root  of  the 
zygoma,  and  below,  by  a  line  drawn  backward  from  the  angle  of  the  jaw. 
While  the  extent  and  outline  of  the  gland  is  somewhat  variable,  its  posi- 
tion is  approximately  that  outlined  in  figure  216.  Its  superficial  surface, 
somewhat  lobulated,  is  in  close  relation  to  the  skin  and  fascia,  while 
deeply  it  penetrates  well  into  the  neck  by  two  processes,  one  of  which 


3°2 


HISTOLOGY 


passes  behind  the  styloid  process  and  beneath  the  mastoid  process  of  the 
temporal  bone  and  the  sternomastoid  muscle,  while  the  other  passes  in 
front  of  the  styloid  process.  Given  off  from  the  body  of  the  gland  and 
extending  in  various  directions  are  a  number  of  processes  or  lobes,  one 
extending  forward  between  the  two  pterygoid  muscles,  and  known  as  the 
pterygoid  lobe;  another  passing  into  the  glenoid  cavity,  the  glenoid  lobe; 
while  a  third  passes  deeply  between  the  carotid  vessels,  and  is  called 


SOCIA  PAROTiDIS 


DUCT  OF  SOCIA 
PAROTIDIS 


DUCT  OF  PAROTID 


Bristle  inserted 
into  duet 


Fraenum  linguae 

DUCT  OF  RIVINUS 


SUBLINGUAL  GLAND 


PAROTID  GLAND 


Masseter  muscle 


DUCT  OF  SUBMAXILLARY 
GLAND 
Mylo-hyoid  muscle 


Anterior  belly  of 
digastric  muscle 


Sterno-mastoid 
muscle 


Posterior  belly  of 
digastric  muscle 


SUBMAXILLARY  GLAND, 
DRAWN  BACKWARDS 


Loop  of  fascia 


DEEP  PORTION  OF  SUBMAXILLARY  GLAND 

Fig.  2 i 6. — The  Salivary  Glands.     (Morris.) 

the  carotid  lobe.  In  many  instances  there  is  an  additional  lobe,  which 
is  detached  from  the  body  of  the  gland,  known  as  the  socia  parotids. 
When  present,  this  lobe  is  placed  over  the  parotid  duct  and  empties  into  it. 
Passing  through  the  substance  of  the  gland  are  a  number  of  arteries  and 
veins,  principal  among  which  are  the  external  carotid,  transverse  facial, 
and  internal  maxillary,  the  gland  receiving  its  blood-supply  by  branches 
from  these.  The  internal  carotid  artery  and  internal  jugular  vein  lie 
close  to  its  internal  surface.  The  facial  nerve  and  its  branches  and  the 
great  auricular  nerve  pass  through  the  gland  from  before  backward,  and 
supply  its  substance  with  nerve-force.  The  weight  of  this  gland  is  from 
one-half  to  one  ounce. 


GLANDS    OF    THE    MOUTH  303 

The  Parotid  Duct. — Leading  from  the  gland  to  the  mouth  is  the 
parotid  or  Stenson's  duct.  After  passing  through  the  fat  of  the  cheek 
and  the  fibers  of  the  buccinator  muscle,  the  duct  comes  in  contact  with  the 
deep  layer  of  the  oral  mucous  membrane.  After  passing  between  this 
structure  and  the  cheek  tissues  for  a  short  distance  it  enters  the  mouth 
opposite  the  crown  of  the  upper  second  molar  tooth,  the  orifice  of  the 
duct  appearing  on  the  surface  of  the  mucous  membrane  in  the  form  of  a 
small  papilla,  which  may  be  readily  observed  with  the  naked  eye.  When 
first  given  off  from  the  gland  a  number  of  small  ducts  are  present,  but 
these  soon  unite  and  form  a  single  canal.  The  parotid  duct  is  quite 
dense,  of  considerable  thickness,  and  is  lined  by  a  reflexion  of  the  buccal 
mucous  membrane. 

The  Submaxillary  Gland. — This  gland,  which  receives  its  name  from 
occupying  a  position  below  the  maxillary  bone,  is  somewhat  smaller 
than  the  parotid.  It  is  situated  beneath  the  mylohyoid  ridge,  and  occu- 
pies the  anterior  part  of  the  submaxillary  triangle,  extending  upward  to 
occupy  the  submaxillary  fossa  on  the  lower  border  of  the  maxilla.  Super- 
ficially, it  is  covered  by  the  skin  and  a  few  muscular  fibers  and  the  deep 
fascia.  The  facial  vein  and  branches  of  the  facial  nerve  pass  over  its 
superficial  surface.  Deeply,  it  is  in  relation  with  the  mylohyoid  and 
hyoglossus  muscles,  and  also  with  the  mylohyoid  artery  and  nerve. 
The  gland  receives  its  blood-  and  nerve-supply  from  the  arteries  and 
nerves  which  penetrate  it.  This  gland  is  separated  from  the  sublingual 
gland  by  the  mylohyoid  muscle,  and  weighs  about  two  drams. 

The  Submaxillary  Duct. — The  submaxillary  duct,  otherwise 
known  as  Wharton's  duct,  passes  forward  and  inward,  opening  into  the 
cavity  of  the  mouth  on  the  summit  of  a  small  papilla  near  the  frenum 
linguae.  The  duct,  which  is  nearly  two  inches  in  length,  first  passes 
through  the  adjacent  muscular  tissue,  and  finally  beneath  the  oral  mucous 
membrane  to  its  outlet.  Like  the  parotid  duct,  it  is  lined  by  a  reflexion 
of  the  oral  mucous  membrane. 

The  Sublingual  Gland.— This  is  the  smallest  of  the  salivary  glands, 
and  is  also  named  from  its  location  beneath  the  tongue.  Its  position  is 
beneath  the  tip  of  the  tongue  and  the  mucous  membrane  covering  this 
part  of  the  floor  of  the  mouth.  It  rests  in  the  submaxillary  fossa  of  the 
maxilla,  meeting  with  its  fellow  at  the  median  line,  and  extending  as 
far  back  as  the  mylohyoid  muscle,  which  separates  it  from  the  sub- 
maxillary gland.  The  gland  is  supplied  with  blood  from  the  sublingual 
and  submental  arteries,  and  with  nerves  from  the  gustatory.  The  weight 
of  this  gland  is  about  one  dram. 


3°4 


HISTOLOGY 


The  Sublingual  Ducts. — This  gland  communicates  with  the  mouth 
by  one  large  duct — the  duct  of  Rivini — which  springs  from  the  main 
portion  of  the  gland,  and  by  a  number  of  smaller  ducts,  eight  to  twenty 
in  number,  which  open  on  the  floor  of  the  mouth.  The  duct  of  Rivini 
follows  the  submaxillary  duct,  and  opens  with  it  at  the  same  papilla. 
The  smaller  ducts  are  "given  off  from  a  number  of  little  lobes  which  cluster 
about  the  fore  part  of  the  gland. 


CHAPTER  VI. 

Glands  and  Ducts  of  the  Mouth;  of  the  Lips;  of  the  Cheeks;  of 
the  Hard  and  Soft  Palates;  of  the  Tongue. — The  Salivary 
Glands. 

GLANDS  AND  DUCTS. 

Glands  of  the  Mouth. — The  glands  of  the  mouth,  like  the  glands 
of  other  parts  of  the  body,  are  composed  almost  entirely  of  epithelium, 
and  may,  therefore,  be  classed  with  the  epithelial  tissues.  Glands  exist 
in  two  principal  forms — tubular  and  saccular  (alveolar).  The  former 
occur  either  singly  or  in  groups,  and  are  further  subdivided  into  simple 
tubular  and  compound  tubular  glands.  A  like  condition  is  present  in 
the  saccular  glands  and  similar  terms  are  employed  to  qualify  them — 
simple  saccular  glands  and  compound  saccular  glands. 

A  simple  tubular  gland  is  one  composed  mainly  of  a  simple  tube- 
like structure;  a  compound  tubular  gland  is  one  composed  of  a  number  of 
smaller  tubes  emptying  into  a  single  duct. 

A  simple  saccidar  gland  is  one  formed  by  a  sacculation  of  serous  or 
mucous  membrane  into  a  single,  simple  sac,  or  by  branched  saccules 
having  an  excretory  duct  (alveolar  system);  a  compound  saccular  gland  is 
composed  of  a  combination  of  branched  saccules. 

In  the  larger  glands  a  sheath  is  formed  by  the  surrounding  connective 
tissue,  from  which  numerous  septa  are  given  off  to  the  interior  of  the 
gland,  dividing  it  into  compartments  varying  in  size.  These  are  known 
as  gland-lobules.  The  connective-tissue  walls  of  the  gland-lobules  carry 
the  larger  blood-vessels  and  nerves.  Most  glands  are  divided  into  two 
essential  parts — the  gland-follicle  and  the  excretory  duct — the  former 
being  specialized  for  the  secretory  function,  while  the  latter,  by  com- 
municating with  the  surface,  conveys  the  secreted  substance  to  that  point. 

The  gland-follicles  are  composed  of  a  layer  of  gland-cells,  usually 
simple  in  character,  surrounding  the  follicular  walls.  External  to  these 
is  a  specially  modified  connective  tissue,  forming  the  basement  membrane, 
or  membrana  propria.  The  appearance  of  the  gland-cells  and  their 
nuclei  is  continually  changing,  being  thus  influenced  by  their  functional 
activity. 

20  3°5 


3°<5 


HISTOLOGY 


The  excretory  ducts  consist  of  a  wall  of  connective  tissue  and  elastic 
fibers,  lined  by  a  columnar  epithelium,  either  simple  or  stratified.  In 
some  instances  the  arrangement  of  the  excretory  ducts  is  much  compli- 
cated, being  divided  into  secretory  tubes,  which  in  turn  are  subdivided 
into  smaller  tubules — intercalated  tubes. 


Fie  217. — Section  through  the  Glandular  Tissue  of  the  Tongue.      X40. 


The  Glands  of  the  Lips  {Labial  Glands). — The  glands  of  the  lips  are 
situated  in  the  submucosa,  and  are  first  observed  immediately  within  the 
line  of  labial  occlusion,  at  which  point  the  thickness  of  the  epithelium 
becomes  somewhat  definite  and  general.  These  glands  are  variable  in 
size,  but  all  are  sufficiently  large  to  be  observed  without  the  aid  of  the 
microscope.  They  are  of  the  compound  tubular  variety,  and  communi- 
cate with  the  surface  through  an  excretory  duct,  which  throughout  the 
greater  part  of  its  extent  is  lined  with  stratified,  scaly  epithelium.  In 
passing  from  the  surface  toward  the  gland-follicle,  the  main  duct  takes  a 
spiral  course  obliquely  through  the  tunica  propria,  and  upon  reaching 
the  submucosa  gives  off  numerous  branches  and  twigs  which  terminate 
in  the  individual  acini.     The  larger  branches  from  the  main  duct  are 


GLANDS  OF  THE  CHEEKS  307 

lined  with  stratified  squamous  epithelium,  while  the  smaller  twigs  are 
provided  with  columnar  epithelium.  In  many  instances  the  main 
excretory  duct,  in  its  passage  through  the  tunica  propria,  receives  the 
principal  duct  from  small  accessory  ducts.  The  framework  of  the  labial 
glands  is  formed  by  the  flexiform  tissue  composed  of  fasciculi  of  the  fine 
connective-tissue  fibers  belonging  to  the  submucous  layer,  together  with 
delicate,  coiled  elastic  fibers.  This  framework  gives  support  to  a  minute 
system  of  capillaries  and  small  nerve-fibers  supplying  the  acini.  The 
acini  are  so  arranged  that  those  belonging  to  a  large  duct  are  united 
into  b  lobule  by  the  submucous  connective-tissue  fasciculi,  and  these  in 
turn  are  formed  into  lobes.  By  a  continuation  of  the  same  fasciculi  and 
fibers  which  limit  a  lobe,  and  in  the  meshes  of  which  the  acini  are  situated, 
a  sheath  to  the  excretory  duct  is  formed.  Besides  the  branched,  tubular, 
mucous  glands  of  the  lips,  there  are  occasionally  found,  at  the  edges  of 
the  lips,  sebaceous  glands. 

The  Glands  of  the  Cheeks  {Buccal  Glands,  Molar  Glands). — The 
glands  of  the  cheeks  are  also  situated  in  the  submucous  layer  of  the  mucous 
membrane.  They,  like  the  labial  glands,  are  of  the  compound  tubular 
variety,  and  when  microscopically  examined  are  found  to  be  similar  in 
structure.  They  are  somewhat  larger  than  the  labial  glands  and  pro- 
portionately less  numerous.  The  chief  duct  from  each  of  these  glands 
usually  opens  with  a  narrow  mouth  on  the  surface  of  the  oral  mucous 
membrane,  and  in  its  passage  through  the  tunica  propria  takes  a  vertical 
or  oblique  direction.  In  the  submucosa  the  chief  duct  branches  into 
two  or  more  smaller  ducts,  taking  up  alveoli.  As  the  buccal  glands  are 
somewhat  larger  than  the  glands  of  the  lips,  they  are  composed  of  a  greater 
number  of  ducts  and  alveoli. 

The  Glands  of  the  Hard  and  Soft  Palate  (Palatal  Glands). — The 
mucous  glands  of  the  hard  palate  are  situated  in  the  submucosa  and 
closely  associated  with  the  periosteum.  They  are  compound  tubular 
glands,  and  in  all  essential  particulars  are  similar  to  the  labial  and  buccal 
glands.  They  are  quite  numerous  (200  to  300),  isolated  in  the  anterior 
portion,  but  are  grouped  into  a  single  row  or  into  two  rows  posteriorly. 
The  glands  are  freely  distributed  in  each  lateral  half,  but  are  absent  at 
the  median  line. 

In  the  soft  palate  the  glands  are  of  the  same  character,  somewhat 
variable  in  size,  the  largest  being  found  in  the  uvula.  The  excretory 
ducts  from  these  glands  vary  in  diameter,  in  the  nature  of  their  fibrous 
structure,  and  in  the  direction  taken  in  passing  to  the  surface.  Over 
the  surface  of  the  soft  palate  the  mouths  of  these  ducts  are  represented 


3°8 


HISTOLOGY 


by  minute  orifices  slightly  smaller  than  the  body  of  the  duct,  but  in  the 
uvula  the  opposite  condition  is  present,  the  mouth  of  the  duct  being  wider 
than  the  body.  The  course  taken  by  the  excretory  duct  is  seldom  a 
direct  one,  but  after  receiving  all  tributary  branches  passes  obliquely 
through  the  tunica  propria,  and  before  entering  the  epithelium  turns 
at  an  abrupt  angle,  and  so  continues  until  the  surface  is  reached.  The 
ducts  are  lined  by  a  simple  columnar  epithelium,  which  in  some  instances 
is  ciliated;  the  walls  of  the  tubes  consist  of  gland-cells  and  a  structure- 
less membrana  propria.  In  some  instances  the  surface  epithelium  may 
be  reflected  for  a  short  distance  and  partly  serve  in  the  capacity  of  a  lining 
to  the  tubular  walls. 

The  Glands  of  the  Tongue  {Lingual  Glands). — In  this  organ  two 
varieties  of  glands   are   found,   occurring   both  in   the  mucous   mem- 


Jp§! 


Fig.  218. — Section  through  Base  of  Tongue,  showing  Serous  Glands. 

brane  and  in  the  superficial  muscular  strata,  being  principally  distin- 
guished by  the  nature  of  their  secretions.  The  gland-cells  of  the  one  set 
are  mucigenous,  secreting  mucin;  these  are  the  mucous  glands.  The 
other  set  is  productive  of  a  serous  fluid,  'thin,  watery,  and  containing 
albumin;  these  are  the  serous  glands. 

The  mucous  glands  of  the  tongue  are  found  along  the  lateral  margins 
and  over  the  root  of  the  organ,  being  most  numerous  in  the  latter  situation. 


THE   SALIVARY   GLANDS  309 

They  are  of  the  compound  tubular  variety,  and  in  most  particulars  are 
identical  with  the  mucous  glands  of  other  parts  of  the  oral  cavity.  The 
ducts  are  lined  with  ciliated  columnar  epithelium,  and  the  walls  of  the 
duct  consist  of  a  homogeneous  membrana  propria  and  gland-cells. 
The  glands  occupying  the  root  of  the  tongue  are  frequently  found  with 
their  excretory  ducts  opening  into  the  follicular  crypts.  The  tubules 
consist  of  a  structureless  membrana  propria  and  numerous  gland-cells, 
the  latter  varying  in  appearance  according  to  their  function  or  functional 
activity.  The  crypts  of  the  follicles  constitute  reservoirs  for  the  acinous 
glands,  and  these  receptacles  frequently  extend  for  some  distance  beneath 
the  surface,  receiving  at  various  points  the  main  excretory  ducts  from 
the  mucous  glands.  These  saccular-like  reservoirs  are  lined  by  a  well- 
defined  capsule  surrounded  by  a  fibrous  sheath,  internal  to  which  is  an 
epithelial  covering,  a  prolongation  of  the  common  epithelium  of  the 
mouth.  Between  these  two  layers  are  a  number  of  minute,  closed  lymph- 
follicles  placed  in  a  single  layer.  The  mucous  glands  on  the  lateral  walls 
of  the  tongue  are,  for  the  most  part,  situated  near  the  middle  or  pos- 
terior portion.  The  ducts  from  these  glands  usually  open  directly  toward 
the  cheek,  but  in  rare  instances  they  pass  obliquely  downward  and  open 
near  the  proper  floor  of  the  mouth.  At  the  tip  of  the  tongue,  buried  be- 
neath the  mucous  membrane  and  some  of  the  muscular  fibers,  may  be 
found  a  pair  of -mucous  glands  (Nuhn's)  which  open  by  free  orifices  on  the 
under  surface.  At  the  root  of  the  tongue,  flat,  lenticulated  elevations  of 
the  mucous  membrane  are  present,  beneath  which  is  imbedded  con- 
globate, glandular  substance.  These  show  a  central  orifice  leading  to  a 
small  pit  lined  with  tessellated  epithelium. 

The  serous  glands  of  the  tongue  are  compound  tubular  glands,  and  are 
found  in  the  region  of  the  circumvallate  papillae,  closely  associated  with 
the  taste-buds.  The  excretory  ducts,  lined  with  a  simple  or  stratified 
columnar  epithelium,  the  latter  sometimes  ciliated,  open  near  the  base  of 
the  papilla,  or  between  the  papilla  and  its  wall.  The  tubules  are  similar 
to  those  in  the  mucous  glands,  consisting  of  a  delicate,  structureless  mem- 
brana propria  and  gland-cells.  The  gland-cells  are  composed  of  a  frail, 
transparent  protoplasm,  containing  rounded  nuclei. 

The  Salivary  Glands. — The  parotid,  submaxillary,  and  sublingual 
glands  each  consists  of  an  excretory  duct,  branching  frequently  in  a  tree- 
like manner  into  smaller  ducts,  lined  throughout  with  a  layer  of  epithelial 
cells.  From  the  smaller  ducts  terminal  branches  are  given  off,  which  in 
turn  are  lined  with  epithelium.  The  other  portions  of  the  glands  are 
invested  by  columnar  epithelium,  and  arranged  like  grapes  about  the 


310  HISTOLOGY 

main  excretory  duct,  and  consequently  belong  to  the  group  of  racemose 
glands.  The  terminal  branches  or  alveoli  attached  to  the  smaller  excre- 
tory ducts  are  so  numerous  that  they  become  much  compressed  from 
pressure,  and  the  grape-like  appearance  is  more  or  less  destroyed,  and 
but  little  space  is  left  for  interstitial  tissue.  Each  gland  is  inclosed  in  a 
fibrous  connective-tissue  capsule,  and  from  this  numerous  septa  of  fibrous 
trabecular  pass  to  the  interior  and  divide  the  glandular  substance,  first  into 
lobes,  these  being  subdivided  into  lobules,  the  lobules  by  further  sub- 
division forming  the  alveoli.  The  glandular  connective  tissue  is  loose 
in  texture,  containing  many  elastic  fibers  and  lymphoid  cells.  Fine 
bundles  of  fibrous  tissue,  together  with  branched  connective-tissue  cor- 
puscles, constitute  the  connective-tissue  matrix  between  the  alveoli. 

The  Ducts. — Entering  the  interior  of  the  gland,  the  chief  duct  divides 
into  a  number  of  large  branches,  one  of  which  passes  to  each  lobe,  each 
of  these  giving  off  several  branches  which  connect  with  the  several  lobules. 
Upon  close  examination  the  central  tube  of  each  lobule  is  observed  to 
throw  off  several  small  tubes — the  intralobular  tubes.  Following  these 
are  the  intermediate  tubules,  which  continue  into  the  terminal  compart- 
ments. The  chief  excretory  duct  consists  of  a  double  layer  of  cylindric 
epithelium  and  fibro-elastic  cartilage.  Close  beneath  the  epithelium  is  a 
compact  membrana  propria.  The  intralobular  tubes  are  each  provided 
with  a  distinct  lumen.  The  walls  are  composed  of  a  membrana  propria 
lined  by  a  layer  of  columnar  epithelium,  the  cells  of  which  contain  a 
central  round  nucleus. 

The  Parotid  Gland. — The  distinguishing  histologic  feature  in  this 
gland  is  found  in  its  excretory  duct  (Stenson's  duct),  which  is  provided 
with  a  membrana  propria,  especially  broad  and  compact,  placed  imme- 
diately beneath  the  epithelium.  The  duct  is  composed  of  a  double 
layer  of  cylindric  epithelium  and  fibrous  tissue,  intermingled  with  elastic 
fibers.  The  main  duct  divides  and  passes  into  the  intralobular  tubes, 
beyond  which  are  the  intermediate  tubules.  The  intralobular  tubes  are 
lined  by  columnar  cells,  while  the  intermediate  tubules  are  lined  by 
elongated,  spindle-shaped  cells.  The  salivary  cells  lining  the  acini  are 
different  in  character  from  those  in  the  submaxillary  and  sublingual 
glands.  The  parotid  gland  is  a  true  salivary  gland,  and  the  serous 
gland-cells  composing  its  epithelial  lining  are  disposed  in  a  single  layer. 
The  cells  are  columnar  or  pyramidal  in  form  and  composed  of  a  dense 
protoplasm,  containing  a  spheric  nucleus. 

The  Submaxillary  Gland. — The  excretory  duct  (Wharton's  duct), 
like  the  main  duct  of  the  parotid  gland,  is  composed  of  a  double  layer 


NERVE-SUPPLY    TO    THE    SALIVARY    GLANDS  3II 

of  columnar  epithelium,  external  to  which  is  a  layer  of  cellular  connective 
tissue,  the  whole  being  surrounded  by  a  thin  stratum  of  muscular  fibers 
placed  longitudinally.  The  intralobular  tubes  are  lined  by  a  specialized, 
elongated,  cylindric  epithelium,  which,  in  the  intermediate  tubules,  be- 
comes clothed  with  cubic  cells.  The  acini  are  lined  either  with  serous 
gland-cells  similar  to  those  lining  the  acini  of  the  parotid  gland  or  with 
mucous  gland-cells,  the  former  being  most  constant  in  their  presence.  The 
two  kinds  of  acini  are  uninterruptedly  connected.  In  most  instances 
there  are  but  a  few  mucous  acini  present  within  the  lobule,  but  occa- 
sionally they  are  found  in  abundance.  The  submaxillary  is  a  mixed 
or  mucosalivary  gland. 

1  The  Sublingual  Gland. — The  excretory  duct  (Rivini's  duct)  is 
similar  in  structure  to  the  chief  excretory  duct  of  the  submaxillary  gland. 
The  intralobular  tubes  are  lined  with  columnar  epithelium.  The  inter- 
mediate tubules  are  not  positively  known  to  exist,  and  it  is  quite  probable 
that  the  intralobular  tubes  pass  directly  into  the  terminal  compartments. 
The  acini  are  composed  of  a  membrana  propria  and  gland-cells,  both 
mucous  and  serous.  The  former  are  much  more  numerous  than  in  the 
acini  of  the  submaxillary.  The  membrana  propria  is  composed  of 
stellate  connective-tissue  cells.  This  gland  is  also  a  mixed  or  muco- 
salivary glands. 

Blood-vessels  and  Lymphatics  in  the  Salivary  Glands. — The 
lobules  of  the  salivary  glands  are  richly  supplied  with  blood-vessels.  The 
many  arterial  branches  break  up  into  numerous  capillaries,  which,  form- 
ing a  dense  network,  surround  the  acini,  being  supported  by  the  inter- 
alveolar  connective  tissue.  The  lymphatic  vessels  accompanying  the 
intralobular  tubes  are  in  communication  with  numerous  lymph-spaces 
which  exist  between  the  interalveolar  connective  tissue  and  the  walls  of 
the  acini.  The  substance  of  the  gland  is  further  supplied  with  blood 
by  numerous  plexuses  of  lymphatics  which  are  carried  or  supported  by 
the  interlobular  connective  tissue. 

Nerve-supply  to  the  Salivary  Glands. — The  nerve-fibers  distrib- 
uted to  the  salivary  glands  are  both  of  the  medullated  and  non-medul- 
lated  variety,  and  other  nerve-tissue  in  the  form  of  ganglion-cells  is 
present.  The  medullated  nerve-fibers  are  abundantly  numerous,  and 
are  distributed  to  all  parts  of  the  gland.  In  many  respects  the  fibers  are 
peculiarly  constructed.  They  are  extremely  delicate,  made  so  by  the 
frail  nature  of  this  medullary  sheath;  they  divide  and  give  off  so  many 
branches  as  to  almost  give  them  a  feathery  fineness.  This  peculiarity 
is  especially  noticeable  toward  their  extremities,  where  the  fibers  lie 


312  HISTOLOGY 

between  the  alveoli  and  give  off  minute  branches  in  all  directions.  The 
nerve-fibers  are  placed  in  close  relation  to  the  tubes  and  tubules,  which 
they  freely  encircle;  they  perforate  the  membrana  propria  and  break  up 
into  finer  subdivisions,  from  which  they  are  distributed  to  the  exterior 
of  the  epithelial  cells.  In  the  alveoli  two  kinds  of  nerve  terminations  are 
found.  The  primitive  fibers  branch  between  the  alveoli  and  are  distrib- 
uted to  the  membrana  propria,  upon  entering  which  numerous  branches 
are  thrown  off  which  pass  to  the  epithelial  cells  beneath.  The  non- 
medullated  fibers,  which  are  much  less  numerous,  are  composed  of  an 
extremely  delicate  fasciculi  of  transparent  fibers  resembling  axis-cylinders, 
and  invested  by  a  sheath  of  connective-tissue  cells  containing  nuclei. 
The  distribution  of  these  fibers  is  similar  to  the  medullated  fibers,  encircling 
the  tubes  and  penetrating  the  membrana  propria,  being  similarly  distrib- 
uted to  the  alveoli. 

Literature. 

Stohr,  "Text-book  of  Histology." 

Strieker,  "Human  and  Comparative  Histology,"  vol.  i,  1870. 

Klein,  "Elements  of  Histology,"  1889. 

Sebastian,     "Recherches    anatomique,    physiologiques,     pathologiques,     les    Glans 

Labiales,"  1842. 
Ward,  "On  Salivary  Glands,"  Tood's  "Cyclopedia  of  Anatomy  and  Physiology.' 
Klein  and  Vernon,  Strieker's  "Human  and  Comparative  Anatomy,"  vol.  i,  chap.  xvi. 
Sudduth,  "Embryology  and  Dental  Histology,"  "American  System  of  Dentistry," 

vol.  i,  part  hi. 
Brubaker,  "Transactions  Odontological  Society  of  Pennsylvania,"  i890-'95. 
Todd  and  Bowman,  "Physiological  Anatomy,"  vol.  i. 
Szontagh,  "Essays  on  Minute  Anatomy  of  Hard  Palate  in  Man,"  1866. 


CHAPTER  VII. 

Tissues  of  the  Teeth — Enamel;  Dentin;  Cementum;  the  Tooth- 
Pulp. — The  Alveolodental  Membrane. 

TISSUES  OF  THE  TEETH. 

Enamel. — The  enamel,  which  forms  a  cap-like  covering  of  varying 
thickness  over  the  entire  crown  of  the  tooth,  is  a  vitreous,  hyaline  sub- 


Fig.  219. — Section  of  Enamel  from  Human  Tooth  (Specimen  by  J.  Howard  Mummery.) 

X350.     {After  Williams.) 

stance,  containing  but  little,  if  any,  organic  matter.     The  thickness  of 
the  enamel  cap  appears  to  be  strongly  influenced  by  the  function  of  the 

313 


314  HISTOLOGY 

different  parts  of  the  tooth-crown,  being  thickest  over  the  cutting-edges 
of  the  anterior  teeth,  while  in  the  cuspidate  teeth,  the  entire  occlusal 
surface  is  provided  with  the  thickest  enamel  layer.  It  is  about  evenly 
distributed  over  the  lateral  walls  of  the  crown,  but  as  the  cervical  line  is 
approached,  its  thickness  is  gradually  diminished  (Fig.  339).  Chemically, 
enamel  is  composed  of  the  salts  of  lime,  calcium  phosphate  predominat- 
ing. Calcium  carbonate,  magnesium  phosphate,  and  calcium  fluorid 
are  present  in  smaller  quantities.  The  proportionate  quantity  of  lime- 
salts  in  enamel  is  not  fixed,  a  slight  variation  in  density  occurring  in  the 
enamel  of  different  individuals.  These  essential  differences  are  regulated 
by  the  proportionate  quantity  of  calcium  phosphate  and  carbonate — a 
greater  amount  of  the  former  being  productive  of  additional  hardness, 
while  an  increase  in  the  latter  beyond  the  minimum  amount  decreases 
this  quality.  As  a  general  rule,  the  teeth  of  males  contain  a  greater 
amount  of  calcium  phosphate  than  the  teeth  of  females,  as  shown  by  the 
following  analysis  by  von  Bibra: 

Man.  Woman. 

Calcium  phosphate  and  fluorid 89 

Calcium  carbonate 4 

Magnesium  phosphate 1 

Other  salts 

Cartilage 3 

Fat 

Total  organic, 3 

Total  inorganic 96 

In  general  structure  enamel  is  composed  of  numerous  hexagonal 
prisms,  with  a  common  direction  at  right  angles  to  the  long  axis  of  the 
tooth.  These  prisms  are  known  as  enamel  prisms,  enamel  fibers,  or 
enamel  rods.  While  the  general  direction  of  the  fibers  is,  as  previously 
stated,  nearly  at  right  angles  to  the  body  of  the  tooth,  they  do  not  pursue  a 
perfectly  straight  course  in  passing  from  the  dentin  to  the  surface,  but 
are  disposed  in  a  tortouus  or  wave-like  manner.  The  enamel  prisms 
may  be  said  to  sit  on  end  against  the  surface  of  the  dentin,  minute  depres- 
sions in  the  latter  receiving  the  extremities  of  the  rods.  The  direction 
of  the  enamel  prisms,  as  compared  to  the  body  of  the  tooth-crown,  varies 
according  to  the  part  of  the  crown  which  they  occupy.  Taking  the 
entire  crown  of  the  tooth,  they  radiate  in  such  a  manner  from  the  surface 
of  the  dentin  that  at  the  cutting-edge  or  occlusal  surface  of  the  tooth  they 
are  more  or  less  vertical,  while  over  the  lateral  surfaces  they  tend  to  the 
horizontal  direction.     An  examination  of  the  prisms  when  isolated  and 


.82 

81.63 

■37 

8.88 

■34 

3-55 

.88 

■97 

•39 

5-97 

.  20 

a  trace 

•59 

5-97 

■41 

94  03 

ENAMEL 


315 


decalcified  exhibits  numerous  evenly  distributed  varicosities,  producing 
a  transversely  striated  appearance  to  the  rods.  Tomes  has  pointed  out 
that  the  enamel  rods  are  variously  disposed  in  that  portion  of  the  enamel 
most  closely  associated  with  the  dentin.  On  the  cusps  of  the  teeth  they 
are  twisted  and  curved  in  various  directions,  while  near  the  surface  on  the 
incisors  they  are  uniform  and  straight.  In  general,  the  enamel  rods, 
which  begin  on  the  surface  of  the  dentin,  are  continuous  through  the 
entire  thickness  of  the  enamel.  In  passing  from  the  interior  to  the  exte- 
rior, the  individual  rod,  to  occupy  a  proportionate  space  in  all  parts, 


Enamel 


Dentin 


Fig.  220. — Enamel  and  Dentin  from  Human  Tooth,  showing  Gradual  Reduction  in  the 
Thickness  of  the  Former  as  the  Cervix  is  Approached.      X20. 


would  have  to  increase  in  diameter;  but  this  it  does  not  do.  In  conse- 
quence of  this  arrangement  there  exist  numerous  supplemental  or  per- 
ipheral rods,  which  extend  but  a  short  distance  from  the  surface,  filling  in 
the  interprismatic  spaces  formed  by  the  longer  rods.  With  the  exception 
of  the  faint  transverse  striations,  the  enamel  prisms  appear  to  be  structure- 
less. A  variety  of  opinions  have  been  expressed  in  regard  to  the  cause 
for  the  striated  appearance  of  the  enamel  rods.  It  was  claimed  by  Hertz 
to  be  attributable  to  a  temporary  arrest  of  calcification,  but  more  recent 
investigation  has  shown  the  cause  to  be  the  presence  of  varicosities  in  the 
individual  fibers,  precisely  as  the  varicosities  in  the  muscular  fibers  pro- 


316 


HISTOLOGY 


duce  the  striated  appearance  in  that  tissue.  Bodecker  asserts  that  fully- 
developed  normal  enamel  is  non-striated,  and  Von  Ebner  practically 
makes  the  same  statement,  claiming  that  they  are  due  to  the  preparation 
of  the  specimen,  which  usually  being  mounted  in  Canada  balsam  is  in- 
fluenced sufficiently  from  the  slight  acid  reaction  to  produce  the  striated 
appearance.  These  statements  Williams  emphatically  denies,  saying 
that,  while  in  some  specimens  the  varicosities  are  apparent  in  some 
parts,  they  are  decided  in  others.  If  this  be  accepted,  it  would  seem 
to  entirely  overthrow  the  theory  of  Von  Ebner  in  regard  to  the  action  of 
the  acid,  which  would  be  distributed  to  all  parts  alike. 


Enamel 


Dentin. 


FlG.  221. — Comparison  in  the  Appearance  of  the  Enamel  and  Dentin  under  Low  Power 

of  the  Microscope.      X40. 

According  to  Williams,  the  varicosities  of  one  enamel  prism  are 
opposite  those  of  the  adjoining  prisms,  and  by  the  coming  together  of 
the  varicosities  the  prisms  become  united  by  means  of  processes  which 
they  send  out.  In  like  manner  the  varicosities  upon  the  same  rod  are 
connected  by  processes  running  parallel  with  the  prism.  According  to 
Von  Ebner,  enamel  is  traversed  by  numerous  minute  canals,  and  Heitz- 
mann  claims  to  have  found  organic  fibers  in  its  substance.  Williams, 
while  admitting  the  enamel  structure  to  be  far  more  complex  than  past 
research  has  shown,  appears  to  have  fully  demonstrated  that  neither  canals 
nor  organic  fibers  are  present — in  fact,  he  denies  the  presence  of  the  least 


ENAMEL 


317 


trace  of  organic  matter  in  this  structure.  The  interprismatic  matrix, 
heretofore  considered  by  most  authorities  to  be  an  organic  structure, 
now  appears,  by  the  thorough  methods  employed  by  the  last-named 
gentleman,  as  a  transparent,  inorganic  substance.  By  numerous  experi- 
ments he  was  enabled  to  secure  a  specimen  in  which  the  interprismatic 
spaces  of  one  layer  were  not  backed  up  by  the  rods  of  another  layer. 
In  some  instances  the  specimen  showed  the  rods  well  separated  and  the 
interspace  closed  by  a  perfectly  transparent  substance,  in  the  interior  of 
which  might  be  seen  connecting  processes  passing  from  one  rod  to  another. 


Enamel 
Rods  Fully 
in  Trans- 
verse Sec- 
tion 


Ename 
Rods  No  t 
Fully   n 
Trans- 
verse 
Section 


Enamel 
Rods  of 
Irregular 
Form 


Fig.  222. — Human  Enamel.     Transverse  Ground  Sections.     {After  Cysi.) 


That  the  enamel  is  practically  of  inorganic  material,  and  therefore  not 
capable  of  transmitting  or  receiving  sensations,  may  be  demonstrated  by 
the  simple  experiment  of  immersing  a  thin  section  of  this  tissue  in  a  weak 
solution  of  chromic  acid,  the  result  of  which  will  be  a  speedy  separation 
of  the  enamel  prisms  which  have  been  liberated  by  the  destruction  of  the 
interprismatic  substance.  What  does  this  signify  ?  Chromic  acid  is 
one  of  the  best  preservatives  of  organic  tissue  known,  and  if  the  cement- 
ing substances  in  enamel  were  organic  or  even  partly  so,  the  prisms  by 
this  test  would  not  be  freed,  but  instead  would  become  more  firmly  ce- 
mented together.     Therefore  we  infer  from  this  that  the  interprismatic 


318  HISTOLOGY 

substance  is  even  less  highly  organic  than  the  prisms  themselves,  these 
not  being  acted  upon  until  after  the  material  which  holds  them  together. 
In  addition  to  the  striated  appearance  formed  by  the  varicosities  of  the 
individual  prisms  of  fully  developed  enamel,  other  structures  of  a  different 
character,  and  upon  a  much  larger  scale,  are  present  and  known  as  the 
" brown  stria  of  Retzius"  (Fig.  223).  These  markings,  readily  seen 
with  a  low  power,  are  of  a  brownish  color,  and  run  nearly  parallel  with 
the  surface  of  the  dentin  or  enamel.  Those  striae  nearest  the  surface  of 
the  dentin  are  inclined  to  follow  the  contour  of  that  structure,  extending 


^fc^WFV^ 


>*■>-%     w8* 


Fig.  223. — Thick  Section  of  Enamel  of  Human  Tooth,  showing  Brown  Striae  of  Retzius. 

X40. 

in  many  instances  the  entire  length  of  the  crown.  The  lines  nearest  the 
surface  are  the  longest  in  the  region  of  the  cutting-edge,  or  occlusal  surface 
of  the  crown,  becoming  shorter  as  the  neck  of  the  tooth  is  approached, 
being  directed  at  an  acute  angle  to  the  surface  of  the  dentin  at  that  point. 
A  number  of  theories  are  advanced  to  account  for  the  presence  of  the 
"brown  striae  of  Retzius."  Tomes  suggests  that,  coinciding  as  they  do 
with  the  outer  surface  of  what  was  at  one  time  the  primitive  enamel  cap, 
they  might  be  considered  as  in  a  measure  outlining  the  stratifications  of 
the  primary  deposit.  Another  theory,  but  one  seemingly  without  founda- 
tion, is  to  the  effect  that  the  striae  are  produced  by  an  arrest  in  the  calci- 


ENAMEL 


3J9 


/ 

j&^llk. 

Brown  striae 
of  Retzius. 

EviM   s 

^5>                       '."^"§|aES5?w 

'■  J|     9 

Lines  of  f 
Schreger.  \ 

TV" 

■-'  ■  -.^'",'1  ■  '-^-  ■i*^CT*/'*  4 

Enamel. 


Dentin. 


Fig.  224. — Enamel  and  Dentin,  Human  Tooth.     {After  Gysi.) 


a    .*- 


wk 


Oral  Epithelium 
Heaped  up 
over  Band 


Band 


Connective  Tissue 


Fig.  225. — Vertical  Section  through  Jaw  of  Human  Embryo.     Formation  of  Tooth-band, 

about  Sixtieth  Day.      X300. 


32° 


HISTOLOGY 


fying  process;  while  a  third  theory  attributes  the  cause  to  a  variation  in 
the  character  of  the  nourishment  taken  by  the  mother  during  pregnancy. 
The  acceptance  of  this  latter  theory  would  seem  to  indicate  a  set  diet 
for  all  mothers  and  at  stated  intervals,  the  striae  always  being  present 
and  somewhat  regularly  distributed  throughout  the  tissue. 

Still  another  set  of  lines  or  markings  are  to  be  observed  in  the  sub- 
stance of  sections  of  enamel,  these  being  known  as  the  "lines  oj Schreger." 
Figure  224  shows  these  lines  as  they  appear  in  the  enamel  by  reflected 
light,  the  same  being  quite  invisible  by  transmitted  light.     The  presence 


Epitheliu 


Band 


Fig.  226. — Vertical  Section,  Tooth-band,  Human  Embryo.     Tenth  Week.      X300. 


of  these  lines  is  due  to  the  various  directions  assumed  by  the  contiguous 
groups  of  enamel  rods.  Beginning  at  the  surface  of  the  dentin  they  are 
well  denned,  but  gradually  become  less  marked  as  the  exterior  of  the 
enamel  is  approached.  At  the  line  of  union  between  the  enamel  and  den- 
tin irregularly  formed  cavities  are  occasionally  observed,  into  which  the 
dentinal  tubules  may  extend,  and  in  rare  instances  individual  tubules  may 
pass  beyond  the  boundary-line  of  the  dentin  and  enter  the  enamel,  but  in 
all  probability  both  of  these  conditions  are  pathologic.  Such  a  state  of 
affairs  could  hardly,  be  considered  normal  when  we  take  into  considera- 
tion that  the  dentin  and  enamel  calcify  in  opposite  directions,  and  that  the 
outer  wall  of  the  former  is  completed  before  enamel  calcification  begins. 


ENAMEL  321 

Development  of  Enamel. — Preparations  for  the  development  of  the 
enamel  begin  toward  the  close  of  the  second  fetal  month,  appearing  first 
as  a  multiplication  of  the  primitive  epithelial  cells  in  the  form  of  a  con- 
tinuous linear  projection  extending  somewhat  obliquely  into  the  sub- 
jacent connective  tissue.  From  this  crest,  or  tooth-band,  the  .germs  for 
the  future  enamel  organs  are  given  off.  These  primary  dental  bulbs, 
as  they  are  called,  number  one  for  each  tooth  to  be  generated,  and  coinci- 
dently  with  their  appearance  an  aggregation  of  closely  associated  connec- 
tive-tissue cells  make  their  appearance  in  the  surrounding  submucous 

Dental  Ridge 


«*2Bfc 


"^F^s^ 


Band 


Location  of  Future 
Enamel  Organ 


**  •  '■ 


'  if' :  "'Si. '"'  '5V*  ■ 


Connective  Tissue  r  k'V  ti'i't+V.-r- 


epithelium 


Fig.  227. — Same  as  Figure  226.     About  Twelfth  Week.      X200. 


tissue.  This  papilla-like  specialization  of  the  submucous  tissue  is  the  prim- 
itive dentin  germ,  or  dentin  papilla.  It  will  thus  be  observed  that  the 
enamel  is  a  product  of  the  surface  epithelium,  ec  to  dermic,  while  the  dentin 
is  generated  from  the  connective  tissues,  mesodermic.  Soon  after  the  ap- 
pearance of  this  club-shaped  thickening,  or  tooth-bulb,  by  further  differ- 
entiation its  form  becomes  bell-shaped,  with  the  concavity  directed  toward 
the  surface.  The  dentin  papilla  gradually  pushes  into  the  concavity 
of  the  forming  enamel  organ,  and  at  a  later  period  the  odontoblastic  cells 
are  generated  about  the  periphery  of  the  papilla,  closely  followed  by  a 
surface  calcification  of  the  dentin.  Soon  a'fter  the  forming  of  the  external 
layers  of  dentin  the  ameloblasts  or  enamel-forming  cells  become  active, 


322 


HISTOLOGY 


and  a  deposition  of  enamel  prisms  takes  place  upon  the  exterior  of  the 
dentin  cap. 

Before  taking  up  the  subject  of  enamel  calcification,  brief  reference 
will  be  made  to  the  further  development  of  the  enamel  organ.  As  the 
growth  of  this  organ  proceeds  there  is,  as  the  result  of  a  rapid  proliferation 
of  the  cellular  structure,  a  marked  tendency  for  the  organ  to  become 
separated  from  the  tooth-band.  The  peripheral  cells  or  external  epithe- 
lium are  columnar  or  prismatic,  and  remain  so,  while  those  in  the  center, 
primarily  polygonal,  soon  become  transformed  into  a  radiating  network 


Epithelium  of  A*. 
Upper  JawiSTS 


Epithelium  a^ 
9 


&J%>  *?*''?  '-<":> ' . 


^S2  *^>'-t:>  &>$*& 


Surface  of  Jaw 


*"S& 


j&Neck  of  Cord 


stellate  Retic- 
ulum of 
Enamel  Organ 


J 


4 
| 

'  Enamel  Organ 

)entin  Papilla 


Forming  Bone 


Fig.  228.- 


-Vertical  Section  through  Tooth-band,  Cord,  Dentin  Papilla,  and  Enamel  Organ, 
about  Fifteenth  Week.      X150. 


or  stellate  reticulum.  The  appearance  of  a  stellate  reticulum  is  first 
observed  to  take  place  in  the  cells  occupying  the  central  portion  of  the 
enamel-organ,  this  cellular  transformation  progressing  from  the  center 
outward,  but  ceasing  before  reaching  the  columnar  surface  cells  contigu- 
ous to  the  dentin  papilla.  Between  the  enamel  cells  and  the  stellate 
reticulum  is  a  layer  of  unaltered  cells — the  stratum  intermedium.  In 
the  earlier  stages  of  the  development  of  the  enamel  organ  the  peripheral 
cells  are  alike,  being  columnar  or  prismatic,  but  almost  coincident  with 
the  appearance  of  the  dentin  papilla,  the  cells  most  closely  related  to  it 
are  observed  to  become  elongated,  and  form  the  internal  epithelium  of  the 
organ.     As  the  cells  forming  this  internal  epithelium  become  elongated, 


ENAMEL 


323 


their  nuclei,  instead  of  occupying  the  center  of  the  protoplasmic  body, 
are  carried  to  their  extremities.  It  will  thus  be  seen  that  the  completed 
enamel  organ  consists  of  four  divisions  or  layers  of  cells.  Beginning 
with  its  convex  surface  is  an  external  epithelium  or  outer  tunic,  succes- 
sively followed,  in  passing  toward  the  dentin  papilla,  by  a  stellate  reticu- 
lum', stratum  intermedium,  and  an  internal  epithelium  or  inner  tunic. 
As  the  growth  of  the  enamel  organ  proceeds,  the  tooth-band  becomes 


/ 


Fig.  229.— Section  of  a  Developing  Tooth  of  Lamb. 
a,  Ameloblasts;  D,  dentin;  P,  dental  pulp. 


X600. 


smaller  and  smaller  in  size,  until  finally  a  complete  rupture  takes  place. 
This  rupture,  however,  does  not  occur  until  the  enamel  organ  has  about 
or  fully  completed  its  development,  and,  after  remaining  so  long  under 
the  influence  of  the  oral  epithelium,  it  must  be  considered,  as  before  stated, 
an  epithelial  structure.  It  is  through  the  agency  of  the  internal  epithelial 
cells  of  the  enamel  organ,  the  enamel  cells  or  ameloblasts,  that  calcification 
of  the  enamel  takes  place,  and  that  subject  will  next  be  considered. 


324 


HIS  IOI.0GY 


Amelijicalion  (Fig.  229). — Two  theories  are  advanced  in  regard  to 
the  calcification  of  the  enamel.  In  one  it  is  claimed  that  the  ameloblasts 
or  enamel  cells  became  directly  calcified  or  converted  into  enamel;  in  the 
other  the  ameoblasts  are  simply  considered  as  controlling  agents,  by 
secreting  or  depositing  the  calcium  salts  which  form  the  enamel  prisms. 
In  the  latter  theory  it  is  generally  believed  that  the  enamel  is  secreted  or 
shed  out  from  the  extremities  of  the  ameloblasts,  thus  being  productive 


m>,. 


Dentin 


Enamel 
Rods 


/     Enamel' 


Amelo- 
blasts 


Section  of 
Papilla? 


Fig.  230. — Section  of  Incisor  of  Rat.      X200.     {After  Williams.) 


of  enamel  fibers  corresponding  in  size  and  position  to  the  secreting  cells. 
By  the  direct  calcification  of  the  ameloblasts,  it  would  be  natural  to 
expect  the  process  to  begin  on  the  exterior  of  the  cell  and  gradually  pass 
into  its  interior,  the  central  portion  being  the  last  to  calcify.  This 
would  result  in  an  enamel  prism  corresponding  in  size  and  form  to  the 
generating  cell,  and  in  a  measure  this  similarity  between  the  calcified 
and  uncalcified  structure  does  exist  and  is  one  of  the  potent  factors 


ENAMEL  325 

in  the  recognition  of  this  theory,  but  it  is  hardly  sufficiently  convincing 
to  warrant  a  general  acceptance  of  the  belief. 

By  examining  figure  229  the  importance  of  the  secretory  theory  is 
favored.  Here  we  note  that  the  early  formed  enamel  at  A  records  what 
appears  to  be  the  definite  action  of  the  ameloblasts  by  prolongations  of 
partly  calcified  tissue  extending  from  the  cells,  these  markings  corre- 
sponding in  number  and  location  to  the  cells  themselves.  Between  these 
prolongations  and  the  ameloblasts  are  many  highly  refractive  granular 
bodies  which  seem  to  be  the  actual  lime  deposit  shed  out  from  the  free 
extremities  of  the  cells,  from  which  they  pass  into  the  substance  of  the 
then  organic  matrix  beyond  at  A  and  form  the  enamel  prisms.  Williams 
takes  exceptions  to  this  latter  view,  substantiating  his  opinion  by  stating 
that  while  the  ameloblasts  of  many  animals  are  similar  in  shape  and 
arrangement,  the  enamel  produced  from  these  similarly  arranged  cells 
varies  greatly  in  structure.  The  same  writer  also  states  that,  when  such 
a  similarity  of  arrangement  exists  between  the  ameloblasts  and  the  enamel 
prisms,  it  occurs  near  the  commencement  of  enamel  calcification,  and 
that  at  a  later  period  the  relative  position  of  the  ameloblasts  and  the 
prisms  is  always  in  longitudinal  section.  Dr.  Williams  calls  attention 
to  the  fact  of  the  enamel  prisms  or  rods  not  extending  through  the  entire 
distance  between  the  enamel  cells  and  the  calcified  dentin  (Fig.  229). 
That  part  of  the  structure  lying  between  the  ameloblasts  and  the  extrem- 
ities of  the  enamel  rods  is  made  up  of  a  double  set  of  fibers,  some  of  which 
are  almost  at  right  angles  with  long  axis  of  the  ameloblasts.  In  figure 
231,  D,  the  two  sets  of  fibers  previously  mentioned  are  found  to  join 
and  become  closely  interwoven. 

The  ameloblasts  are  connected  with  the  cells  of  the  stratum  inter- 
medium, and  more  recent  investigation  goes  to  prove  that  this  latter 
structure  is  directly  interested  in  furnishing  to  the  ameloblasts  the  proper 
material  for  the  calcifying  process.  The  stratum  intermedium  cannot, 
however,  take  part  in  the  primary  enamel  calcification,  as  this  process 
commences  before  the  stratum  intermedium  is  fully  developed.  This 
being  the  case,  it  is  generally  supposed  that  the  stellate  reticulum  furnishes 
the  material  for  the  upbuilding  of  the  first  enamel  prisms. 

Lying  between  the  free  extremities  of  the  ameloblasts  and  the  enamel 
in  the  course  of  formation  is  what  has  been  generally  considered  as  a 
structureless  basement  membrane  or  membrana  prcrformativa.  The 
existence,  exact  location  and  structure  of  this  membrane  has  been  and 
still  remains  a  matter  of  conjecture.  The  generally  accepted  theory 
appears  to  be  that  given  above,  but  Williams  refers  to  it  as  a  layer  of 


326 


HISTOLOGY 


newly  formed  enamel,  and  does  not  consider  it  as  a  structureless  mem- 
brane. The  structure  is  to  be  observed  in  figure  231,  at  either  extremity 
of  the  ameloblastic  cells,  this  writer  claiming  that  the  so-called  structure- 
less membrane  is  present  at  both  of  these  points. 

As    to    the    formation    of   the   enamel  rods,   Dr.  Andrews,  in    1894, 
referring  to  the  presence  of  calcoglobulin  in  the  enamel  cells  considered 


Ameloblasts 


Position  of  Capillary  Loop 


Fll 


231. — Section  of  Incisor  of  Rat,  showing  Partial  Decalcification  of  Enamel. 

{After  Williams.) 


X600. 


these  refractive  bodies  as  calcospherites,  which,  after  being  taken  up  by 
the  ameloblasts,  were  excreted  by  them,  and  after  coalescing  formed 
globules  of  larger  size,  from  which  the  rods  were  built  up.  Williams 
partly  agrees  with  this  statement,  but  he  is  of  the  opinion  that  the  calco- 
spherites coalesce  while  in  the  ameloblasts,  forming  large,  spheric  bodies, 
but  that  the  deposit  of  this  substance  is  in  no  way  productive  of  building 


ENAMEL 


327 


the  enamel  rods.  The  theory  of  Tomes  in  regard  to  the  forming  of 
enamel  rods  was  that  the  walls  of  the  ameloblasts  themselves  became 
calcified,  while  the  contents  of  the  cells  also  became  solidified,  the  first 
forming  the  interprismatic  substance,  while  the  second  became  the 
enamel  rod.  Whatever  theory  be  accepted  as  to  the  formation  of  the 
enamel  rods  or  prisms,  there  appears  to  be  no  question  in  regard  to  the 


Fig.  232. — Section  of  Developing  Tooth  of  Embryo  Lamb.      X150.     {After  Williams.) 

a,  Forming  Dentin;  b,  Forming  Enamel;  c,  Stratum  Intermedium;  d,  Inner  Ameloblastic 

Membrane;  e,  Outer  Ameloblastic  Membrane;/,  Ameloblasts. 


general  process  of  enamel  calcification.  In  the  first  place,  an  organic 
matrix  is  formed,  into  which  the  first-formed  layer  of  enamel  is  disposited. 
Gradually  the  organic  matter  disappears,  leaving  behind  the  inorganic 
elements  closely  resembling  in  appearance  the  organic  matrix,  which 
it  has  by  atomic  change  supplanted.     The  question  of  an  organic  inter- 


328 


HISTOLOGY 


prismatic  cement-substance  is  also  one  upon  which  various  writers  dis- 
agree. Klein  partly  believes  that  such  a  substance  does  exist,  basing 
his  opinion  upon  the  fact  that  the  ameloblastic  cells,  in  common  with  all 
epithelial  cells,  are  separated  from  one  another  by  a  homogeneous  inter- 
cellular substance,  and  that  a  certain  proportion  of  this  organic  substance 
must  remain  between  the  enamel  prisms  after  calcification.  Dr.  Sudduth, 
by  a  series  of  experiments  made  some  years  ago,  appeared  at  that  time 


Fig.  233. 


to  have  furnished  conclusive  proof  that  an  organic,  interprismatic  cement- 
substance  does  not  exist  between  the  enamel  prisms  of  fully  developed 
enamel.  By  the  use  of  a  dilute  solution  of  chromic  acid,  the  action 
of  which  is  the  preservation  of  organic  substance,  the  prisms  were  liberated, 
which  would  not  have  been  the  result  had  they  been  cemented  by  an 
organic  cement-substance.  By  substituting  dilute  muriatic  acid,  the 
action  of  which  is  the  destruction  of  organized  tissue,  the  prisms  were  not 


ENAMEL  329 

liberated,  the  acid  acting  evenly  upon  the  whole  mass  of  enamel,  and 
finally  resulting  in  its  complete  destruction,  not  leaving  the  slightest  trace 
of  an  organic  matrix  behind.  Dr.  Williams  claims  that  this  transparent 
cement-substance  is  formed  by  the  distribution  of  a  translucent  liquid 
substance  about  the  previously  formed  pattern  for  the  enamel  rods. 
This  pattern,  generated  through  the  activity  of  the  enamel  cells,  is  com- 
posed of  a  translucent  material  somewhat  more  solid  than  that  substance 
which  surrounds  it.  These  two  substances  calcify  together,  the  latter 
forming  the  enamel  prisms,  while  the  former  creates  the  cement  or  inter- 
prismatic  substance.  There  is  no  better  method  by  which  to  study  the 
character  or  mode  of  development  of  a  growing  or  matured  tissue  than 
by  an  artificial  disassociation  of  its  component  parts.  In  enamel  it 
matters  but  little  in  what  part  of  the  tissue  or  at  what  period  of  its  growth 
the  examination  be  made  to  learn  of  the  action  of  the  decalcifying  agent; 
it  will  never  be  found  to  take  place  in  a  manner  corresponding  to  the 
direction  of  the  enamel  fibers,  but  decalcification  takes  place  in  older 
enamel  more  in  the  form  of  a  general  breaking  up  of  the  structure,  as 
shown  at  A  in  figure  233,  while  at  B,  which  represents  the  newly  formed 
tissue,  the  action  is  one  which  appears  to  indicate  a  breaking  up  of  the 
interprismatic  substance,  favoring  the  theory  of  secretory  amelification. 

This  distinction  between  the  enamel  cells  and  their  product  possesses 
none  of  the  characteristics  to  properly  classify  it  as  structureless.  There 
is  but  little  doubt  as  to  its  character,  being  the  primary  product  of  the 
ameloblasts,  while  the  corresponding  zone  at  the  distal  end  of  the  ena- 
mel-forming cells  results  from  the  functional  activity  of  the  stratum 
intermedium. 

Figure  242  shows  a  section  near  the  point  of  the  cusp  of  a  developing 
molar  and  exhibits  a  portion  of  the  enamel  organ  at  a  time  immediately 
prior  to  the  beginning  of  enamel  calcification.  A  is  the  uncalcified  dentin, 
B  the  ameloblasts,  now  closely  associated,  and  a  very  regular  layer  of 
elongated  cells,  and  behind  these  another  layer  of  cells,  which  undoubt- 
edly serve  as  feeders  to  the  ameloblasts,  the  stratum  intermedium.  This 
section  is  especially  valuable  in  that  it  shows  a  number  of  capillaries 
distributed  through  the  body  of  the  stellate  reticulum  and  actually  pene- 
trating the  stratum  intermedium,  as  seen  at  B. 

When  viewed  with  a  low  power,  these  minute  blood-vessels  appear 
to  form  a  complete  network,  and  in  the  district  between  the  cusps  pervade 
the  entire  structure,  from  the  stratum  intermedium  on  one  side  to  the 
same  cells  on  the  other.  The  appearance  of  this  animated  vascular  sup- 
ply to  the  enamel  is  coincident  with  the  process  of  calcification,  for  during 


330 


HISTOLOGY 


the  early  life  of  the  tooth-germ  it  is  non-vascular.  The  growth  of  enamel, 
strata  upon  strata,  from  within  outward  is  therefore  by  the  direct  cal- 
cification of  the  enamel  cells  or  ameloblasts,  and  while  this  is  going  on,  and 
as  long  as  the  crown  of  the  tooth  is  incased  in  its  epithelial  cap,  the  enamel 
organ,  the  growth  of  the  tissue  is  stimulated  through  the  blood-vessels 


Fig.  234. 


everywhere  present  in  the  stellate  reticulum.  The  presence  of  this  special- 
ized blood  supply  to  the  central  portion  of  the  enamel  organ  was  for  a  long 
time  doubted,  but  at  present  it  can  be  readily  observed  (Fig.  233).  As 
soon,  however,  as  the  tooth  passes  through  the  surface  tissue,  carrying 
with  it  the  external  epithelium  of  the  enamel  organ  as  the  enamel  cuticle, 
the  possibility  of  nourishment  has  been  cut  off,  and  after  a  little  time  it 


DENTIN 


33* 


becomes  a  petrified  dental  epithelium,  no  longer  nourished  and  absolutely 
non-vital. 

Dentin. — This  tissue,  which  constitutes  the  principal  bulk  of  the 
hard  part  of  the  tooth,  forms  a  complete  cap-like  investment  over  the 
pulp,  from  which  it  is  generated.  It  is  white  or  slightly  yellowish-white 
in  color,  somewhat  elastic,  and  a  trifle  harder  than  bone,  which  it  resem- 
bles in  many  of  its  characteristics.  In  a  perfectly  developed  tooth  no  part 
of  the  dentin  appears  upon  the  surface,  that  part  within  the  crown  being 
covered  by  the  enamel,  while  that  of  the  root  is  inclosed  by  the  cementum. 


Ename 


Dentin 


Fig.  235. — Section  through  Crown  of  Human  Cuspid.      X30. 


While  the  thickness  of  the  dentin  varies  somewhat  over  the  different  parts 
of  the  tooth,  there  is  a  decided  disposition  to  an  equal  distribution  in 
every  direction.  Dentin,  unlike  enamel,  consists  of  an  organic  matrix — ■ 
a  reticular  tissue  of  fine  fibrils  richly  impregnated  with  the  salts  of  cal- 
cium, in  this  resembling  the  matrix  of  bone.  Traversing  the  matrix 
are  long,  fine  canals  or  tubes  (Fig.  243) — the  dentinal  tubules — which 
pass  from  the  margins  of  the  pulp  toward  the  surface.  Immediately 
surrounding  the  dentinal  tubules  the  matrix  is  especially  dense,  forming 
a  lining  or  sheath  to  the  tubes,  known  as  the  dentinal  sheaths.  Occupying 
the  lumen  of  the  dentinal  tubules  are  solid  elastic  fibers — the  dentinal 
fibers.     Dentin,   therefore,  presents  for  examination,   first,  the   matrix; 


332  HISTOLOGY 

second,  the  dentinal  tubules;  third,  the  dentinal  sheaths;  and  fourth, 
the  dentinal  libers. 

The  Matrix. — As  previously  stated,  the  matrix  is  composed  of  organic 
and  inorganic  substances,  but  the  proportionate  quantity  of  organic 
and  inorganic  constituents  is  so  variable  that  it  is  impossible  to  furnish 
a  definite  chemic  analysis.  The  relative  quantity  of  organic  and  in- 
organic matter  is  not  only  variable  in  the  teeth  of  different  individuals, 
but  is  continually  changing  in  the  teeth  of  the  same  individual,  the 
former  being  present  in  larger  quantities  during  youth  and  gradually 


Fig.    236. — Section   through   Root  of   Human   Incisor,   showing   many   Dentin   Tubules   in 

Transverse  Section.      X200. 

diminishing  as  age  advances.     From  an  examination  of  perfectly  dried 
dentin,  the  following  approximate  analysis  has  been  obtained: 

Organic  matter  (tooth-cartilage) 27.61 

Fat 0.40 

Calcium  phosphate  and  fiuorid 66.72 

Calcium  carbonate 3-3^> 

Magnesium   phosphate 1.08 

Other  salts o .  83 

The  organic  basis  of  the  matrix  appears  to  be  structureless  and 
transparent,  and,  although  closely  resembling  the  matrix  of  bone,  is 
not  identical  with  it.     While  the  matrix  is  usually  structureless,  there 


DENTIN 


333 


are  instances  in  which  the  presence  at  one  time  of  connective-tissue  fibers 
is  indicated. 

The  Dentinal  Tubules  (Figs.  237,  238,  239). — Beginning  by  a  free 
opening  about  the  walls  of  the  pulp-cavity,  the  dentinal  tubules  permeate 
the  matrix  in  all  directions.  The  tubules  are  generally  disposed  in  a 
direction  perpendicular  to  the  surface,  so  that  in  different  parts  of  the 
tooth  they  radiate  in  various  directions.  Beginning  upon  the  surface 
of  the  pulp-cavity,  at  which  point  they  are  of  greatest  diameter,  they 
pass  more  or  less  in  a  spiral  manner  toward  the  surface  (Fig.  239),  before 
reaching  which  they  become  gradually  reduced  in  size,  as  a  result  of  the 


Lacuna 


Dentinal  Tubules 


Cementum 


Dentin 


Fig.,  237. — Dentin  and  Cementum  from  Root  of  Human  Molar.    (After  Gysi.) 

numerous  branches  which  they  give  off  (Fig.  238).  The  branches  given 
off  from  the  main  tubes  are  quite  variable  in  size,  and  anastomose  with 
one  another  or  with  the  branches  from  other  tubules.  In  the  region  of 
the  pulp  the  tubules  are  so  closely  associated  that  but  little  space  is  pro- 
vided for  the  intertubular  substance  or  matrix;  but  as  the  surface  is 
approached  they  become  widely  separated,  and,  in  consequence,  the 
matrix  substance  is  present  in  greater  abundance.  While  the  general 
direction  of  the  tubes  is  perpendicular,  they  do  not  pursue  a  direct  course, 
but  are  more  or  less  curved  as  they  pass  from  within  outward.  The 
curvature  of  the  tubuli  may  be  divided  into  two  classes — long  curves 
and  short  curves — usually  referred  to  as  the  primary  and  secondary  curva- 
tures of  the  dentinal  tubules.  The  primary  curvatures  are  few  in  number 
and  are  most  prominent  in  the  crown,  while  the  secondary  curvatures,  prin- 


334 


HISTOLOGY 


cipally  found  in  the  roots,  are  smaller  and  more  numerous.  The  branches 
from  the  main  tubes  terminate  in  various  ways,  either  by  anastomosis, 
by  gradually  fading  out  into  hair-like  terminals,  or  by  ending  in  hooks 
and  loops.  In  rare  instances  they  are  said  to  enter  the  substance  of 
the  enamel  or  cementum,  but  it  is  doubtful  if  they  do  so  normally.  The 
branches  from  a  main  tube  are  usually  two  in  number,  the  latter  being 
almost  equal  in  diameter  to  the  former,  and  from  this  first  set  of  branches 
a  number  of  minute  branches  are  given  off  almost  at  right  angles.  In 
the  crown  this  latter  class  of  tubules  are  seldom  observed,  excepting 
near  the  enamel  margin,  but  in  the  root  they  are  everywhere  noticed. 


Fig.  238. — Longitudinal  Section  through  Root  of  Human  Molar.     Branching  of  the  Dentinal 

Tubules.     X200. 


Small  varicosities  are  frequently  present,  but  not  in  sufficient  numbers 
to  produce  a  striated  appearance  on  the  surface  of  the  dentin. 

The  Dentinal  Sheaths. — While  the  dentinal  tubules  ramify  through 
the  matrix  in  the  form  of  well-defined  channels,  the  walls  of  the  channels 
are  not  formed  by  the  matrix,  but  by  an  indestructible  substance  the 
exact  character  of  which  is  not  fully  understood.  The  walls  of  the  tubes, 
or  the  dentinal  sheaths,  as  they  are  termed,  are  believed  by  some  histolo- 
gists  to  be  calcified,  while  others,  though  acknowledging  their  apparent 
indestructibility,  are  doubtful  as  to  the  correctness  of  this  theory.  Neu- 
mann being  the  first  to  accurately  describe  the  walls  of  the  tubules,  they 
have  become  known  as  "Neumann's  sheaths."     The  existence  of  the 


DENTIN 


335 


dentinal  sheaths  may  best  be  demonstrated  by  subjecting  the  tissue  to 
the  action  of  strong  acid  for  a  sufficient  time  to  destroy  the  intervening 
matrix,  which  process  usually  requires  several  days.  The  fibrous  mass 
remaining  will  be  found  to  contain  a  collection  of  tubes,  which,  however,  by 
careful  examination,  are  found  not  to  be  the  dentinal  tubules  themselves, 
but  the  walls  of  these  canals.  Magitot  and  Sudduth  deny  the  existence  of  a 
wall  to  the  dentin  tubes.  Tomes,  while  inclined  to  the  belief  that  the  tubes 
are  provided  with  definite  walls,  suggests  that  they  may  have  been  pro- 
duced artificially  during  the  preparation  of  the  specimen,  and  that  they 
are  only  brought  into  existence  by  the  action  of  the  agent  used  for  this 


Fig.  239. 


-Transverse  Section  through  Root  of  Human  Molar,  showing  the  Curvature  of 
the  Dentin  Tubules  about  the  Pulp-canal.      X40. 


purpose.  In  conclusion,  the  same  writer  adds  that  that  part  of  the 
matrix  immediately  surrounding  the  fibril  differs  in  its  chemic  con- 
stituents from  the  body  of  the  matrix. 

The  Dentinal  Fibers. — Occupying  the  lumen  of  each  dentin  tube  is 
a  soft,  elastic  fiber,  which  is  continuous  with  and  has  its  origin  from  the 
odontoblastic  cells  upon  the  periphery  of  the  pulp.  The  existence  of 
these  elongated  processes  of  the  odontoblasts  having  first  been  demon- 
strated by  Tomes,  they  are  otherwise  known  as  Tomes'  fibers.  By 
means  of  these  fibers,  which  not  only  fill  the  lumen  of  the  larger  tubes, 
but  the  minute  branches  as  well,  the  substance  of  the  dentin  is  both 


33^ 


HISTOLOGY 


nourished  and  rendered  slightly  sensitive.  There  is  still  some  doubt  as 
to  the  real  nature  of  the  fibrils,  but,  if  they  are  processes  from  the  odonto- 
blasts, it  would  appear  that  the  substance  would  be  identical  with  that 
of  the  cell-protoplasm.  Bodecker  claims  that  they  are  not  round  but 
inclined  to  angularity,  but  Tomes  infers  that  this  form  has  been  produced 
by  the  action  of  some  reagent.  Klein  advances  the  theory  that  the  odon- 
toblasts are  active  in  the  generation  of  the  matrix  for  the  dentin  only, 
and  that  the  dentinal  fibrils  are  not  processes  from  them,  but  originate 
from  cells  intervening  between  the  odontoblasts  and  connecting  with  the 
dentin  tubes.     It  has  never  been  fully  demonstrated  that  true  nerve- 


X&Z 


Fig.  240.— Longitudinal  Section  through  Root  of  Human  Tooth,  showing  Primary  Curvature 
of  Dentin  Tubules.      X40. 

fibers  enter  the  dentin  along  with  or  in  the  substance  of  the  dentinal 
fibril,  but,  while 'the  evidence  is  not  at  present  forthcoming,  there  is  but 
little  doubt  that  the  sensitiveness  of  the  dentin  is  produced  by  the  presence 
of  organized  tissue  in  the  tubuli.  Some  contend  that  the  contents  of  the 
tubules  are  made  of,  first,  a  creative  portion,  that  given  off  directly  from 
the  odontoblasts;  second,  a  circulatory  portion,  a  minute  vessel  traversing 
each  tubule,  entering  either  by  the  side  of  the  cell-bodies  or  passing 
through  them,  and  that  the  nerve  terminals  are  distributed  in  the  same 
manner.  Others  say  that  minute  nerve-filaments  from  the  pulp  pass 
directly  through  the  odontoblasts  and  are  continued  in  the  center  of  the 
tubule  surrounded  by  a  simple  connective  tissue,  the  cell  process,  and 


DENTIN  337 

that  in  this  way  sensations  are  conveyed.  It  is  now  generally  conceded 
that  dentin  is  a  highly  organized  connective  tissue;  that  it  has  a  circula- 
tory system  and  is  endowed  with  sensation  to  a  slight  degree;  that  these 
conditions  are  brought  about  not  by  actual  entrance  into  the  tubules  of 
separate  vessels  and  nerve-filaments,  but  more  in  the  way  of  the  tubules 
being  occupied  by  a  general  connective-tissue  substance  resembling  in 
all  essential  features  the  pulp  itself,  being  the  semi-fluid  interfibrillar 
ground-substance  of  the  pulp;  that  dendrites  of  sensory  neurons  every- 
where present  in  the  pulp,  after  losing  their  medullary  sheaths  divide 
into  fine  varicose  fibers  and  become  closely  associated  with  the  peripheral 

Cemen-  .--re- 

turn      |  .       v.  "* 


Dentin 


iljp 


Pulp-canal 


-r:  * 


*ye0^^S^/- 


Fig.  241. — Transverse  Section  through  the  Root  of  a  Human  Incisor,  showing  the  Dentin 
Surrounded  by  the  Cementum.      X30. 

cells,  pass  between  these,  and  enter  the  cone-shaped  openings  of  the 
tubules  and  terminate  soon  after  doing  so. 

While  the  microscope  reveals  in  some  instances  what  appear  to  be 
prolongations  from  the  dentinal  fibers  penetrating  the  enamel,  or  between 
its  prisms,  such  a  condition  is  improbable  if  not  impossible.  If  this 
arrangement  is  present  at  all,  it  is  so  slight  as  to  have  no  influence  what- 
ever over  the  enamel  either  as  to  nourishment  or  sensation.  No  conclu- 
sions can  be  drawn  with  positive  certainty  from  sections,  since  the  slightest 
deviation  from  parallelism  in  the  surfaces  may  easily  produce  deceptive 
appearances.  It  is  just  as  common,  and  even  more  so,  to  find  hair-like 
lines  interwoven  and  running  parallel  with  the  surface  of  the  dentin 


33& 


HISTOLOGY 


immediately  between  this  tissue  and  the  enamel,  as  it  is  to  see  slight 
fibers  crossing  beyond  their  boundary-line  to  penetrate  the  enamel. 
The  most  likely  place  of  all  to  find  such  a  condition  would  be  in  the 
beginning  of  calcification,  and  here  it  is  never  observed.  The  peripheral 
pulp-cells,  usually  all  classed  as  odontoblasts,  are  never  found  outside 
their  own  territory,  the  dentinal  papilla;  but  their  location  in  the  beginning 
on  the  very  "surface  of  the  papilla,  almost  in  direct  contact  with  the  inner 
tunic  of  the  enamel  organ,  would  make  it  possible  for  their  processes, 
when  appearing,  to  penetrate  between  the  cells  of  the  enamel  organ  if 
they  were  grown  out  from  the  bodv  of  the  cells  from  which  they  spring. 


Cementum 


Dentin 


Granular 
Layer 


Fig.  242. — Tomes'  Granular  Layer.      X40. 


This,  however,  they  do  not  do.  They  do  not  grow  out  from  the  cell- 
body,  so  to  speak,  but  the  cell  recedes,  leaving  them  behind.  By  this 
arrangement  the  terminals  of  the  future  fibers  become  definitely  estab- 
lished, all  increase  in  length  taking  place  in  the  opposite  direction,  toward 
the  pulp.  While  the  active  enamel-forming  cells  are  present  some  little 
time  prior  to  the  odontoblasts,  calcification  of  the  enamel  does  not  take 
place  until  after  a  definite  cap  of  dentin  has  been  formed,  imprisoned  in 
which  are  the  terminal  branches  of  the  fibers.  Therefore  the  fact  that 
this  cap  of  dentin  is  formed  first,  and  this  is  not  a  question  in  dispute, 
with  the  fibers  or  cell  processes  securely  encapsulated  within  it,  would 


DENTIN  339 

seem  to  be  sufficient  evidence  to  qualify  the  statement  that  the  dentinal 
fibers  do  not  penetrate  the  enamel.  The  examination  of  very  many 
sections  of  young  growing  teeth  exhibits  the  fact  that  the  early  formed 
dentin  and  enamel  will  separate  bodily,  leaving  a  positive  clear  line  of 
separation  and  a  surface  absolutely  devoid  of  anything  resembling  the 
prolongation  of  the  fibers  extending  from  the  surface  of  the  dentin. 

Interglobular  Spaces.- — In  that  part  of  the  dentin  which  immediately 
underlies  the  cementum  numerous  intercommunicating,  irregularly 
branched  spaces  are  found.  These  are  known  as  the  interglobular  spaces 
(Fig.  242).  On  account  of  the  granular  appearance  which  this  portion 
of  the  dentin  exhibits  under  low  magnifying  power,  Tomes  has  desig- 


Fig.  243. — So-called  Interglobular  Spaces  in  Dried  Section  of  Dentin.      Xioo. 


nated  it  as  the  "granular  layer."  The  granular  layer  is  also  found  upon 
that  portion  of  the  dentin  which  underlies  the  enamel,  but  in  this  region 
it  is  far  less  marked.  Many  of  the  dentin  tubes  have  their  endings  in 
these  spaces.  While  the  interglobular  spaces  are  most  numerous  near 
the  peripheral  portion  of  the  dentin,  they  are  by  no  means  confined  to  these 
parts.  They  are  present  in  all  parts  of  the  dentin,  but  not  so  closely 
associated,  and  may  be  observed,  when  a  dried  section  of  dentin  is  exam- 
ined, as  spaces  with  irregular  outlines  and  sharp-pointed  processes  ex- 
tending in  various  directions  (Fig.  243).  The  term  "interglobular  spaces" 
becomes  partly  a  misnomer  when  the  so-called  "spaces"  are  more  care- 
fully examined.  In  normal  dentin  the  "spaces"  are  filled  with  a  soft, 
living  plasma,  having  a  structural  arrangement  similar  to  the  general 


34© 


HISTOLOGY 


matrix  of  the  dentin,  and  it  is  only  in  a  dried  specimen  that  a  true  space 
is  found  by  the  shrinking  or  shriveling  of  the  organic  contents.  The 
interglobular  spaces  forming  the  granular  layer,  which  are  much  more 
numerous,  but  of  smaller  size,  than  those  found  in  the  body  of  the  dentin, 
are  also  filled  with  a  soft  living  plasma,  and  they  communicate,  on  one 
hand,  with  the  dentinal  fibers,  and,  on  the  other,  with  the  lacunae  and 
canaliculi  of  the  cementum.  According  to  Sudduth,  the  interglobular 
spaces  (so  called)  are  occupied  by  masses  of  calcoglobulin  which  have 
not  become  fully  calcified. 

N  Dentinification. — The  dentin  bulb,  or  papilla  from  which  the  dentin 
is  forced,  having  already  been  described  in  Part  I,  the  process  of  calcifi- 


Pulp-cells 


Odontoblasts    v?i?  .  o*- 


Odontoblasts 


'.  Calcified  Dentin 


Uncalcified  Dentin 


Fig.  244 — Pulp  and  Forming  Dentin  from  an  Incisor  Tooth.     (After  Gysi.) 


cation  will  at  once  be  taken  up.  It  will  be  recalled  that  calcification  of 
the  dentin  does  not  begin  until  the  dentin  papilla  has  developed  to  the 
form  and  size  of  the  dentin  of  the  future  tooth-crown.  When  this  has 
taken  place,  there  is  generated  upon  the  surface  of  the  papilla  a  modified 
form  of  connective-tissue  cells  called  odontoblasts  (Fig.  244).  These 
cells,  which  are  arranged  in  a  single  row  upon  the  exterior  of  the  papilla, 
vary  in  form  according  to  their  activity.  When  most  active,  they  are 
broadest  at  the  extremity  directed  toward  the  interior  of  the  papilla. 
Proceeding  from  a  single  odontoblast  there  may  be  one  or  more  processes, 
which  are  supposed  to  eventually  occupy  the  tubes  of  the  dentin,  as  the 
dentinal  fibers.  These  cells  each  contain  an  oblong  nucleus,  which  occu- 
pies the  extremity  of  the  cell  most  distant  from  the  dentin,  but  during 


DENTIN" 


341 


the  period  of  greatest  activity  becomes  elongated  or  pointed  in  the  direc- 
tion of  the  process.  The  odontoblastic  cells,  while  actively  engaged  in 
the  calcifying  process,  are  closely  asso:iated  or  crowded  together,  but 
previous  to  this  time  there  is  more  or  less  space  between  them,  which  is 
filled  with  an  indifferent  tissue.  The  first  layer  of  dentin  being  formed 
upon  the  surface  of  the  papilla,  it  will  be  observed  that  all  additions  to 
its  bulk  take  place  from  within  (the  reverse  being  true  of  enamel). 


Fig.  245. — Section  through  Pulp  and  Forming  Dentin. 


As  stated  elsewhere,  calcification  of  the  dentin  begins  upon  the 
coronal  extremities  of  the  crowns,  the  cutting-edges  of  the  incisors  and 
cuspids,  and  the  summits  of  the  cusps  in  the  cuspidate  teeth  first  receiving 
their  lime-salts.  While  the  odontoblasts  undoubtedly  superintend  the 
calcifying  process,  the  part  taken  by  these  cells  appears  to  be  somewhat 
indefinitely  determined.  It  is  generally  supposed  that  the  li>ne-salts  are 
secreted  under  the  superintendency  of  the  odontoblasts.  The  secretion, 
however,  does  not  take  place  around  the  cells,  and  in  that  way  completely 


342  HISTOLOGY 

encapsule  them,  but  around  their  fibrils.  While  this  is  taking  place  the 
odontoblasts  remain  free  upon  the  surface  of  the  pulp,  and  the  fibrils 
assume  their  places  as  the  organic  dentinal  fibrils.  As  the  body  of  dentin 
becomes  thicker,  the  odontoblasts  are  forced  to  recede,  and  in  so  doing 
the  fibers  lengthen.  The  dentinal  tubules  are,  of  course,  formed  in  a 
like  manner,  the  walls  of  the  tube  being  first  calcified  from  the  secretion 
of  lime-salts  by  the  fibrils;  and  as  the  fibrils  lengthen  by  the  increasing 
thickness  of  dentin  and  the  receding  of  the  odontoblasts,  the  tubes  also 
lengthen. 

The  general  character  of  the  pulp  cells  at  a  time  immediately  prior 
to  the  appearance  of  the  ameloblasts  is  vastly  different  from  the  same 
cells  at  maturity  or  after  calcification  of  the  dentin  has  taken  place.  The 
reason  for  this  is  obvious,  considering  that  the  connective-tissue  mass 
does  not  assume  its  principal  function  until  the  odontoblasts  are  generated 
about  its  periphery.  While  at  a  later  period  the  cells  of  the  pulp  are 
oblong  in  shape,  with  slender  tail-like  processes  given  off  from  each  end, 
we  find  the  same  cells  in  the  early  embryo  (sixteenth  to  twentieth  week) 
spheroidal  in  outline  and  distributed  as  they  continue  to  be,  at  irregular 
intervals  about  the  semi-gelatinous  matrix.     (See  Fig.  245.) 

When  the  periphery  of  the  pulp  is  reached,  a  definite  layer  of  cells  is 
present,  corresponding  in  every  particular  to  those  of  the  interior,  and 
it  is  from  these  spheric  bodies  that  the  dentin-forming  cells  are  derived. 
While  the  odontoblasts  are  usually  characterized  as  spindle-  or  flask- 
shaped  cells,  this  can  only  apply  to  the  cells  of  later  life,  as  those  active 
at  the  beginning  of  calcification  do  not  partake  of  either  of  these  forms. 
Figure  246  (twentieth  to  twenty-fourth  week)  shows  the  first  formed 
odontoblasts  actively  engaged  in  their  function  of  dentinification.  It  will 
be  observed  that  the  cells,  instead  of  being  individualized  as  they  appear 
at  a  later  period,  now  present  a  racemose  arrangement,  such  a  cluster 
appearing  about  the  entrance  to  each  dentinal  tubuli,  which  at  this  period 
are  widely  separated  and  apparently  without  anastomosing  branches. 
The  nearer  the  summit  of  the  crown  is  approached,  the  less  apparent  is 
this  grape-like  association  of  the  cells,  showing  conclusively  that  it  is 
a  primary  condition. 

After  a  definite  thickness  of  calcified  dentin  appears  about  the  surface 
of  the  tooth-pulp,  the  character  of  the  odontoblastic  cells  becomes  mate- 
rially changed  (twenty-fourth  week),  but  even  yet  they  do  not  answer 
to  the  description  accorded  them.  The  elongated,  spindle-shaped  or 
club-shaped  odontoblasts  are  without  question  found  in  connection  with 
the  tissue  only  after  calcification  has  progressed  to  a  considerable  extent; 


DENTIN 


343 


and  while  it  does  not  appear  possible  to  detect  the  minute  processes  which 
penetrate  the  calcifying  structure  before  this  stage  of  the  phenomena  has 
been  reached,  they  have  nevertheless  existed  from  the  earliest  inception 
of  this  specialized  layer  of  cells. 

In  this  connection  the  query  presents  itself  in  regard  to  the  manner 
in  which  the  intercommunication  between  the  dentinal  fibers  is  established, 
and  the  probable  cause  for  the  so-called  interglobular  spaces  about  the 


,-*•«■  .  :yj  y  *  • 

4k'                    .  uim    gL 

_~rUT. 

^»w?  ^avr                      ^ 

— J)t»>*»»». 

—  •       •  •  *** .  ■!*£i£M^;lSSs& 

^F^HI             r 

*v3^^                      r 

^'^BeSS®*^^ 

Fig.  246. — Young  Odontoblasts  Attached  to  Forming  Dentin. 


periphery  of  the  dentin.  The  former  can  probably  be  explained  by  an 
examination  of  the  peripheral  cells  of  the  pulp  at  a  time  immediately 
prior  to  the  beginning  of  calcification,  when  it  will  be  found  that  these 
primitive  odontoblasts  communicate  with  one  another  in  a  manner  quite 
similar  to  the  canaliculi  between  the  lacunae  of  the  true  bone,  the  connect- 
ing processes  being  encapsuled  within  the  substance  of  the  calcifying 
tissue. 

Figure  247  is  taken  from  a  very  thin  section  of  a  growing  tooth  at 


344 


HISTOLOGY 


a  time  in  which  we  would  most  naturally  look  for  the  appearance  of  the 
interglobular  spaces,  and  many  such  imperfections,  if  they  be  so  classified, 
arc  observed  within  the  substance  of  the  newrly  formed  tissue. 

Exceptions  might  be  taken  to  the  statement  concerning  the  racemose 
appearance  of  the  early  odontoblasts  previously  referred  to,  by  claiming 
the  section  to  be  one  not  directly  through  the  long  axis  of  the  cells,  or 


Fig.  247. — Section  through  Crown  of  Growing  Tooth  of  Lamb. 

perhaps  transversely  through  them.  The  examination  of  a  number  or 
sections,  one  of  which  is  shown  in  figure  247,  shows  the  cells  cut  trans- 
versely; the  forming  dentin  at  a  appears  with  the  tubuli  squarely  cut  off, 
showing  the  dentinal  fibers  confined,  or  rather  appearing  as  though  pro- 
jecting from  the  lumen  of  the  tubes.  It  will  also  be  noted  that  the  odonto- 
blasts are  irregular  in  outline,  some  of  them  being  almost  hexagonal,  and 


DENTIN  345 

as  the  calcified  tissue  is  approached,  they  gradually  become  reduced  in 
size  and  much  modified  in  contour. 

After  a  dentin  cap  or  matrix  of  considerable  thickness  has  made  its 
appearance  and  the  enamel  cells  are  about  to  assume  their  functional 
activity,  the  odontoblasts  for  the  first  time  begin  to  resolve  themselves 
into '  the  elongated  flask-shaped  cells,  thus  answering  the  description 
usually  accorded  them,  as  illustrated  in  figure  244.     In  fact  it  would 


Fig.  248. — Odontoblasts  in  Transverse  Section. 

appear  that  they  assume  this  shape  only  when  the  actual  lime  deposit 
begins.  If  we  examine  the  line  of  union  between  the  dentin  and  enamel 
during  the  early  growth  of  these  tissues,  it  will  be  ascertained  that,  not- 
withstanding the  dissimilarity  of  the  two  structures  at  maturity,  there 
appears  at  this  line  of  junction  a  matrix  which  may  be  defferentiated  only 
by  the  free  extremities  of  the  ameloblasts,  as  shown  in  figure  249.  When 
the  enamel  matrix  begins  to  form,  a  faint  line  of  demarcation  between 
the  two  may  be  observed,  the  difference  in  the  appearance  of  the  ground- 


346 


HISTOLOGY 


work  of  the  two  structures  being  one  brought  out  by  differential  staining, 
that  of  the  enamel  taking  the  darkest  stain. 

It  will  be  observed,  therefore,  that  after  the  dentin  germ  has  assumed 
the  exact  size  of  the  dentin  of  the  future  tooth,  certain  cells  appear  upon 
its  periphery,  and  under  their  superintendence  a  definite  layer  of  dentin 


Fig.  249. — Section  through  Pulp,  Dentin,  and  Forming  Enamel. 
A,  Calcified  dentin;  B,  Pulp;  C,  Ameloblasts. 

soon  results.  This  first  formed  layer  of  dentin  is  definite  and  unchange- 
able in  location,  and  it  has  within  its  substance  the  minute  processes  from 
the  dentin -forming  cells  which  are  destined  to  become  and  really  are  the 
terminals  of  the  dentinal  tubules.  All  who  have  given  the  subject  of 
dentin  calcification  careful  consideration  are  practically  agreed  as  to  the 
part  which  the  peripheral  pulp-cells  play  in  the  process.     This  is  to  the 


DENTIN  347 

effect  that  not  about  the  body  of  the  cells  themselves,  but  around  their 
processes  the  lime  salts  are  deposited.  After  a  distinct  layer  of  specialized 
cells  has  become  fully  established  upon  the  very  periphery  of  the  papilla, 
the  first  change  which  takes  place  is  a  slight  withdrawal  of  these  cells 
from  this  point,  leaving  behind  slender  hair-like  processes  which  occupy  a 
portion  of  the  space  previously  taken  up  by  them,  and  about  the  ex- 
tremities of  the  cells  and  their  processes  which  are  directed  toward  the 
enamel  organ  calcified  material  is  generated.  Fone  upon  zone  of  calcified 
dentin  appears  in  this  way,  the  body  of  the  cell  receding,  leaving  in  its 
wake  its  processes  encapsuled  within  the  calcified  structure  as  the  dentinal 
fibers. 

In  connection  with  the  primitive  layer  of  dentin-forming  cells,  there 
are  usually  described  lateral  processes  passing  from  cell  to  cell,  apparently 
serving  the  purpose  of  communication  between  the  cells.  But  these 
have  recently  been  shown  to  be  simply  a  network  of  connective-tissue 
fibers  supporting  the  body  of  the  cells.  The  theory  of  Andrews,  brought 
out  some  years  ago  in  regard  to  the  specialized  layer  of  pear-shaped 
cells — dentin  corpuscles,  as  he  termed  them — may  be  accepted,  and 
these  should  be  considered  as  having  something  to  do  with  the  process  of 
dentinification.  The  presence  of  these  pear-shaped  cells  at  the  beginning 
of  calcification  and  during  the  continuance  of  this  process  can  be  easily 
demonstrated,  and  if  we  accept  them  as  being  concerned  in  the  process 
of  dentin  formation,  they  might  in  a  measure  modify  the  function  now 
accorded  the  elongated  club-shaped  cells,  the  odontoblasts.  It  is  ques- 
tionable whether  the  odontoblasts  alone  are  responsible  for  the  growth 
of  dentin;  they  undoubtedly  control  the  actual  process  of  lime  deposit, 
but  the  additional  cells,  no  doubt,  contribute  to  the  structural  make-up 
of  the  tissue.  It  may  be  that  by  modifying  certain  parts  of  the  matrix, 
the  result  in  the  general  structure  is  the  dentinal  sheaths;  this  part  of  the 
tissue  being  so  markedly  different  from  the  bulk  of  the  intercellular  sub- 
stance would  lead  us  to  believe  that  it  was  developed  from  specialized 
cells.  Further,  it  is  said  that  while  the  dentinal  tubules  are  filled  with 
a  living  substance,  this  substance  is  not  solely  the  product  of  the  processes 
of  the  odontoblasts.  That  there  is  a  special  distribution  of  non-medul- 
lated  nerve  terminals  as  well  as  a  rich  plexus  of  blood-vessels  about  the 
periphery  of  the  pulp  is  unquestionable,  and  this  supply  is  just  as  plentiful, 
or  perhaps  more  so,  at  maturity  as  it  is  at  the  beginning  of  calcification 
when  the  dentin  cells  are  most  active.  From  this  we  might  be  led  to 
believe  that  this  special  blood  and  nerve  supply  to  the  periphery  of  the 
pulp  is  not  solely  for  the  upbuilding  of  the  dentin  and  therefore  distributed 


348 


HISTOLOGY 


to  the  peripheral  cells,  but  also  for  the  permanent  welfare  of  the  resultant 
tissue,  this  being  brought  about  by  some  circulatory  system  throughout 
the  tubules  of  the  dentin. 

Cementum  (Fig.  250). — Investing  the  roots  of  the  teeth  is  a  sub- 
stance which,  both  chemically  and  physically,  is  closely  allied  to  bone. 
This  external  covering  is  known  as  the  cementum,  and  while  generally 
regarded  as  being  confined  to  the  roots  of  the  teeth,  by  some  it  is  considered 


Lucun; 


Fig.  250. — Cementum  from  Root  of  Molar.      X200. 


to  extend  to  and  completely  invest  the  crowns  during  the  early  part  of 
their  existence,  in  this  latter  location  being  known  as  the  enamel  cuticle, 
or  membrane  of  Nasmyth. 

Generally  speaking,  the  cementum  begins  by  a  thin  margin  at  the 
neck  of  the  tooth  or  cervical  line.  It  may  commence  at  the  free  enamel 
margin  of  the  crown,  or  it  may  slightly  overlap  this  structure.  It  is 
thinnest  at  the  neck  of  the  tooth,  and  gradually  increases  in  thickness  as 
the  apex  of  the  root  is  approached.     In  teeth  with  closely  associated 


CEMENTUM 


349 


roots  the  cementum  frequently  extends  from  one  root  to  the  other,  result- 
ing in  a  firm,  osseous  union.  Histologically  considered,  the  structure  of 
cementum,  like  ordinary  bone,  consists  of  a  gelatinous,  basal  substance, 
combined  with  the  salts  of  lime,  and  of  numerous  little  hollow  spaces — 
lacuna.  Branching  in  every  direction  from  the  lacunae  are  many  minute 
processes — canaliculi. 

The  Matrix. — The  matrix  is  so  nearly  identical  with  that  of  bone 
that  it  is  with  difficulty  that  they  can  be  distinguished.  By  decalcification 
it  retains  its  form  and  structure,  and  by  the  intimate  blending  of  organic 


Fig.  251. — Longitudinal  Section  through  Root  of  Human  Molar. 
Incremental  Lines  of  Cementum.      X30. 

and  inorganic  substances  it  is  provided  with  hardness,  solidity,  and 
elasticity.  Calcium  salts  and  collagenous  fibrils,  united  by  a  small 
amount  of  cement-substance,  in  finer  or  coarser  bundles,  compose  the 
ground-substance,  or  matrix,  of  cementum. 

Let  us  first  take  up  the  study  of  this  tissue  at  different  periods  of  its 
existence,  and  in  this  manner  learn  of  its  character,  its  mode  of  develop- 
ment, and  the  changes  which  take  place  as  its  growth  proceeds.  The 
striated  markings  of  the  tissue  have  led  to  the  belief  that  there  are,  during 
the  process  of  cementification,  periods  of  activity  and  periods  of  rest  or 
little  activity.  An  examination  of  the  structure  under  low  power  (Fig 
251)  shows  the  incremental  lines  placed,  with  more  or  less  regularity,  one 


35° 


HISTOLOGY 


beyond  the  other,  and  when  thus  studied  adds  much  to  the  strength  of  the 
theory  of  interrupted  development. 

Figure  252  is  prepared  from  a  developing  deciduous  incisor  three 
months  after  birth.  At  this  period  the  devloping  organ  is  made  up  of 
enamel  and  dentin  alone,  the  process  of  cementification  not  yet  being 
under  way.  The  establishment  of  the  dentinal  periphery,  which  surface 
is  unchangeable,  provides  a  basis  for  the  first  layer  of  cementum  generated 
by  the  cementoblasts,  which  at  this  period  are  forming  about  the  inner 
wall  of  the  tooth-follicle.     In  close  proximity  to  the  surface  the  inter- 


Enamel 


^ 


Surface  of  I 
Dentin 


•  r"  J 


W 


Fig.  252. — Section  through  Developing  Incisor,  Three  Months  after  Birth.     X30. 


globular  spaces  are  observed  somewhat  widely  distributed,  and  propor- 
tionately large  in  size,  resulting  in  a  surface  poorly  calcified  and  forming  a 
ready  attachment  for  the  cemental  tissue.  Figure  253  shows  the  process 
of  cementification  under  way,  the  section  being  prepared  from  a  six- 
month-old  tooth.  In  an  examination  of  the  ground-substance  of  this 
developing  tissue  there  is  an  unbroken  granular  appearance,  possessing 
neither  striations,  fibers,  nor  cement-corpuscles.  This  appearance  is 
one  which  persists  in  the  oldest  or  first-formed  stratum,  and  is  again 
noticeable  in  the  outermost  or  youngest  stratum.  While  the  oldest 
stratum  or  strata  retain  this  primary  character,  this  cannot  be  said  ^of 


CEMENTUM 


351 


those  subsequently  laid  upon  it,  for  they  successively  develop  in  their 
matrix  the  partially  calcified  cells  and  fibers  from  the  formative  tissue. 

Figure  254,  taken  from  a  one-year-old  tooth,  shows  a  further  advance 
in  the  process  of  cementification.  Many  of  the  transverse  fibers  of  the 
alveolodental  membrane  are  observed  penetrating  the  developing  tissue, 
and  will,  at  a  later  period,  by  their  partial  calcification,  become  a  part  of 
its  substance.  Already  there  has  been  established  an  intimate  blending 
of  the  cemental  tissues  with  the  dentinal  tissues  through  the  medium  of  the 
granular  layer,  and  by  the  further  calcification  of  the  latter  this  union 


Fig.  253. — Developing  Cementum,  from  Six-month-old  Tooth.      X200. 
a,  Developing  Cementum;  b,  Granular  Layer;  c,  Terminals  of  the  Dentinal  Tubuli. 


gradually  becomes  more  thorough.  Figure  255  illustrates  three  distinct 
zones  of  developing  cementum;  the  older  unbroken  granular  zone  at  A, 
now  beautifully  cemented  to  the  granular  layer;  a  second  or  intermediate 
zone,  B,  having  encapsuled  within  its  ground-substance  many  of  its 
formative  cells;  and  an  outer  zone,  C,  but  recently  laid  down,  showing 
numerous,  longitudinal,  wave-like  striations,  emblematic  of  the  cemento- 
blastic  activity.  In  this  outer  zone  the  minute  laminations  disappear 
as  the  tissue  becomes  more  thoroughly  calcified  and  the  matrix  gradually 
partakes  of  the  nature  of  the  older  tissue. 


352 


HISTOLOGY 


The  position  occupied  by  the  cementum  on  the  root  has  much  to  do 
with  its  character.     In  the  region  of  the  cervix  the  cement-corpuscles 


Fibers  of 
Cementum 


Older  Strata 


^J^Granular 
^F     Layer 


Fig.  254. — Section  through  One-year-old  Tooth.      X60. 


V 

4? 

T 

■ 


A 


;*N^>Cv 


c<*-» 


Fig.  255. — Developing  Cementum,  from  Transverse  Section  of  Bicuspid.      X  100. 


are  few  in  number,  and  when  present  possess  extremely  short  and  irregular 
processes.     In  the  region  of  the  apex  the  structure  is  much  more  complex 


CEMENTUM 


353 


in  character,  longitudinal  striae,  transverse  fibers,  cement-corpuscles, 
and  zones  of  apparently  unbroken  granular  matrix  all  serving  to  this  end. 

To  continue  the  study  of  this  tissue  let  us  examine  in  detail  the 
lamellae,  the  cement-corpuscles,  and  the  cement-fibers. 

The  Lamella. — We  are  told  that  the  lamellae  are  about  the  same  in 
number  over  all  parts  of  the  tooth-root,  but  that  they  are  much  thinner 
at  the  neck  than  at  the  apex.  In  addition  to  this  they  are  usually  con- 
sidered as  running  parallel,  or  nearly  parallel,  to  the  surface  of  the  dentin. 
While  these  statements  might,  and  probably  do,  describe  the -disposition 


MS 


Fig.  256. — Transverse  Section  from  Root  of  Bicuspid,  showing  Variation  in  the 
Disposition  of  the  Lamellae.      X40. 

of  the  lamallae  in  young  cementum,  they  do  not  apply  with  so  much 
certainty  to  the  conditions  after  the  adult  period.  The  lamellae  in  the 
region  of  the  apex  are  not  only  of  greater  width,  but  are  usually  greater 
in  number  than  those  occupying  the  cervix  of  the  same  root. 

Figure  256  is  prepared  from  the  transverse  section  of  an  adult  bicuspid 
in  the  region  of  the  apex,  and  shows  how  the  disposition  of  the  lamellae 
may  vary  in  thin,  normal  cementum.  At  A,  which  represents  the  granular 
union  of  the  cementum  with  the  dentin,  the  incremental  lines  are  ob- 
served to  follow  the  surface  of  the  dentin.  As  the  center  of  the  area  is 
approached  this  regularity  is  much  interfered  with,  some  of  the  lamellae 
being  discontinued,  others  greatly  thickened,  while  the  field,  taken  in  its 
23 


354 


HISTOLOGY 


entirety,  exhibits  anything  but  regularity  in  the  laying  down  of  the 
different  strata.  This  same  condition  may  be  observed  in  longitudinal 
section.  While  the  lamellae  are  usually  characteristic  of  the  cemental 
tissue  in  general,  they  are  seldom  found  in  interdentinal  cementum,  or 
that  growth  which  takes  place  between  roots,  resulting  in  their  fusion 
(Fig.  257).  This,  of  course,  refers  to  the  tissue  as  formed  between  closely 
associated  roots  of  an  individual  tooth,  and  not  to  that  union  which 
sometimes  takes  place  between  the  roots  of  different  teeth.  The  inter- 
dentinal tissue  previously  referred  to  appears  to  have  many  characteristics 


Interdentinal 
Cementum     f 


Fig.  257. — Transverse  Section  through  Fused  Roots  of  Molar  Tooth,  showing 
Interdentinal  Cementum.      X30. 


common  to  itself;  thus,  the  cement-corpuscles  are  peculiar  in  form,  fibers 
are  few  in  number,  and,  as  before  stated,  the  lamellae  are  not  decided. 
Cement-corpuscles. — Many  of  the  cementoblasts  of  the  peridental 
membrane,  like  the  osteoblasts  of  the  periosteum,  become  encapsuled 
within  the  developing  tissue,  and  persist  as  irregularly  shaped  spaces, 
filled  with  a  protoplasmic  mass,  and  are  known  as  cement-corpuscles. 
These  correspond  to  the  lacuna,  of  bone,  but,  unlike  these,  are  very  variable 
in  size,  in  form,  and  in  the  number  and  direction  of  their  processes. 
Figure  250  shows  a  number  of  cemental  lacunae  and  canaliculi.  In 
the  majority  of  instances  the  body  of  the  corpuscle  will  be  found  to  be 
oval  or  slightly  oblong,  with  its  long  axis  parallel  to  the  surface;  but  it  is 
by  no  means  uncommon  to  find  them  very  irregular  in  outline,  with  the 


CEMENTUM 


355 


greatest  diameter  in  the  opposite  direction.  The  processes  are  quite 
variable  in  length  and  irregular  in  their  course,  and,  while  there  is  a 
general  disposition  for  them  to  extend  toward  the  surface,  they  in  many 
instances  radiate  in  various  directions.  All  of  these  features  are  in  con- 
tradistinction to  the  lacunae  and  canaliculi  of  bone,  which  are  placed 
with  much  more  regularity  in  the  osseous  matrix,  the  corpuscles  being 
oblong  or  cylindric  in  outline,  with  their  processes  about  equally  distrib- 
uted in  every  direction,  and  uniting  directly  and  positively  with  the 
canaliculi   of  neighboring  lacunae.     As  previously  stated,   the  cement- 


Fig.  258. — Cement-corpuscles  of  Outer  or  Younger  Strata.     X40. 


corpuscles  are  very  variable  in  outline,  this  difference  in  form  appearing 
to  be  much  influenced  by  the  part  of  the  tooth  examined.  The  younger 
corpuscles  (Fig.  258),  or  those  associated  with  the  outer  strata,  are  usually 
distinctly  outlined  and  provided  with  delicate  processes,  the  majority  of 
which  are  directed  toward  the  surface.  In  the  older  strata  the  outlines 
of  the  corpuscles  are  much  more  irregular,  the  processes  short  and  ex- 
tremely clumsy. 

The  proportionate  distribution  of  the  corpuscles  to  the  various  parts 
of  the  tooth-root  is  as  follows:  The  innermost  or  oldest  zone  and  the 
outermost  or  youngest  zones  contain  but  few;  in  the  intervening  strata 
they  are  most  abundant,  especially  in  the  region  of  the  apex,  becoming 
less  numerous  in  passing  crownward.     In  interdentinal  cementum  the 


356 


HISTOLOGY 


corpuscles  are  somewhat  regularly  distributed  throughout  the  ground- 
substance  adjacent  to  the  granular  layer,  but  near  the  center  of  this 
confused  mass  of  imperfectly  calcified  tissue  they  are  seldom  present. 
When  the  interdentinal  space  is  slight,  peculiarly  formed  corpuscles  are 
often  observed  (Fig.  259),  provided  with  a  long,  rod-like,  central  portion 
or  trunk,  from  which  are  given  off  numerous  tree-like  branches,  the 
terminals  of  which  are  frequently  lost  in  the  granular  layer  upon  either 
side. 

Cement-fibers. — In  a  manner  similar  to  that  in  which  the  cemento- 
blasts  become  encapsuled  within  the  developing  cemental  tissue  forming 


Fig.   259. — Cement-corpuscles  Common  to  Interdentinal  Cementum.      X  ioo. 

the  cement-corpuscles,  many  of  the  fibers  of  the  peridental  membrane 
undergo  a  like  transformation,  and  are  found  in  the  tissue  as  more  or  less 
imperfectly  calcified  fibers  transversely  disposed.  By  many  writers  these 
filamentary,  thread-like  structures  have  been  compared  to  the  delicate, 
net-like  processes  which  pass  through  the  concentric  lamellae  of  bone, 
serving  to  hold  them  together  and  designated  as  Sharpens  fibers;  but, 
according  to  Black,  these  are  the  principal  fibers  of  the  alveolodental 
periosteum,  and,  as  already  stated,  become  a  part  of  the  cemental  tissue 
during  its  evolution.  In  figure  260  the  fibers  are  shown  under  high 
power;  A  represents  the  primary  or  older  stratum  of  the  tissue,  and  it  is 
from  the  outer  margin  of  this  zone  that  the  fibers  first  make  their  appear- 
ance, passing  more  or  less  directly  in  the  direction  of  the  surface  until  the 


CEMENTUM  357 

next  incremental  line  is  reached,  at  which  point  they  gradually  disappear, 
but  recur  in  the  succeeding  lamellae.  There  is  a  marked  disposition  for 
the  fibers  of  each  concentric  lamella  to  keep  within  its  borders,  or,  in 
other  words,  to  become  individualized;  but  in  many  instances  they  pass 
through  from  one  lamella  to  another,  and  occasionally  extend  unbroken 
through  the  entire  thickness  of  the  tissue.  It  occasionally  happens  that 
the  fibers  are  plentifully  distributed  to  a  region  comprising  three  or  four 
lamella?,  followed  by  a  zone  of  similar  proportions  in  which  they  are 
entirely  absent.  The  cement-fibers,  considered  as  the  partially  calcified 
residue  of  the  principal  fibers  of  the  peridental  membrane,  would  naturally 


ft 

1 

< 

.  V 

•>; 

Fig.  260. — Transverse  Section  through  Root  of  Molar,  showing  Cemental  Fibers.      X300. 

assume  a  general  direction  relative  to  their  manner  of  distribution  before 
this  change  had  taken  place,  and  in  most  instances  they  are  thus  disposed, 
In  figure  261,  taken  from  the  center  of  a  long  axis  of  a  growing  bicuspid, 
the  disposition  of  the  fibers,  which  are  alone  observed  in  the  second 
lamella,  is  slightly  crownward.  The  inclination  for  the  fibers  to  be  thus 
disposed  is  most  pronounced  in  young  cementum,  but  after  middle  life, 
or  at  a  period  when  the  tissue  has  greatly  increased  in  thickness,  the 
course  of  the  fibers,  even  in  the  same  locality,  is  greatly  at  variance. 

In  figure  254,  also  from  a  young  tooth,  the  fibers  are  shown  springing 
directly  from  the  peridental  membrane,  with  their  free  extremities 
penetrating  this  tissue.  This  illustration  is  prepared  from  a  transverse 
section  in  the  cervical  region,  and  the  inclination  of  the  fibers  is  such  as 


358 


HISTOLOGY 


to  warrant  the  belief  that  they  were  some  of  those  whose  function  it  has 
been  to  return  the  tooth  to  its  normal  position  when  slightly  rotated  upon 
itself.  Another  class  of  fibers  common  to  the  cement-tissue  are  those 
which  appear  to  be  grouped  in  bundles,  springing  more  or  less  regularly, 
at  intervals,  from  the  granular  layer  and  penetrating  the  basement  layer 
of  the  cementum  as  though  serving  to  tie  this  tissue  to  the  periphery  of 
the  dentin.  In  figure  262  a  number  of  these  bundles  are  shown  at  A,  B, 
and  C.  While  the  field  is  but  a  small  proportion  of  the  circumference 
of  the  root,  they  are  observed,  under  low  power,  to  be  distributed  in  a 


Fig.  261. 

like  manner  to  all  parts.  These  circumferential  fibers,  as  they  may  be 
called,  are  also  observed  in  longitudinal  section,  being  distributed  with 
considerable  regularity  throughout  the  whole  extent  of  the  root.  They 
are  also  present  in  the  tissue  at  the  earliest  period  at  which  its  character 
may  be  studied,  the  individual  bundles  at  this  time  being  proportionately 
larger.  These  might  be,  and  probably  are,  considered  as  prolongations 
from  the  dentinal  fibers,  but  it  is  doubtful  if  the  true  fibers  of  the  dentin 
are  ever  found  penetrating  the  cementum. 

Cementification. — We  have  seen  in  the  study  of  the  development  of 
the  teeth,  that  the  tooth-generating  organs  were  confined  in  a  closed  sac 
or  follicle,  and  while  the  walls  of  this  sac  were  not  directly  interested  in 
the  calcification  of  the  dentin  or  enamel,  this  cannot  be  said  of  the  cemen- 
tum.    Attention  has  also  been  directed  to  the  fact  that  at  the  time  of 


THE    DENTAL    PULP 


359 


the  eruption  of  the  crown  of  the  tooth  a  portion  of  the  root  only  is  cal- 
cified. As  the  growth  of  the  root  continues,  the  follicular  wall  becomes 
closely  adherent  to  it.  Upon  the  inner  face  of  this  vascular  membrane 
a  layer  of  osteoblastic  cells  (cementoblasts)  is  generated,  and  as  a  result 
of  the  calcification  of  these  cells  the  cementum  is  formed.  It  will  thus  be 
seen  that  the  process  of  cementification  is  but  a  slightly  modified  form  of 
subperiosteal  bone  development.  At  the  beginning  of  cementum  cal- 
cification the  diameter  of  the  dentin  of  the  root  is  as  great  as  it  will  ever 
be,  all  additions  to  its  bulk  taking  place  from  within.     But  while  the 


diameter  of  the  dentin  is  thus  fixed,  the  diameter  of  the  root  is  increased 
by  the  additional  layers  of  cementum  as  they  are  deposited  upon  its 
surface.  As  previously  stated, 'a  single  layer  of  cementoblasts  is  first 
formed  in  the  membrane  surrounding  the  root,  these  soon  becoming 
inclosed  in  a  spherule  of  lime.  By  the  time  this  has  taken  place  another 
layer  makes  its  appearance,  assuming  all  the  characteristics  of  the  first 
formed  layer.  Other  layers  are  formed  in  turn  until  the  cementum 
assumes  its  mature  thickness. 

The  Dental  Pulp  (Fig.  263.). — The  tooth-pulp,  or  formative  organ 
of  the  dentin ,  occupies  the  central  or  pulp-cavity,  and  in  the  fully  developed 
tooth  assumes  a  general  outline  closely  corresponding  to  the  exterior 


36° 


HISTOLOGY 


of  the  organ.*  Along  with  its  primary  function  of  generating  the  dentin, 
it  becomes  the  medium  through  which  this  structure  receives  its  vascular 
and  nervous  supply. 

Histologically  considered,  the  pulp  may  be  described  as  a  mucous- 
like,  protoplasmic  matrix,  containing  delicate  connective-tissue  fibers 
not  formed  into  bundles  and  numerous  nucleated  cells,  the  latter  being 
especially  numerous  on  the  periphery  of  the  pulp,  or  that  portion  which 
comes   in  contact  with  the  dentin.     The  cells  are  not  closely  enough 


Enamel 


Cementum 


Dentin 


Pulp 


Blood-vessels 


Fio.  263. — Longitudinal  Section  through  Human  Cuspid,  showing  Tooth-pulp. 
(After  Gysi.)     X  10. 


associated  to  form  a  complete  tissue  in  themselves,  but  are  found  em- 
bedded in  a  mucoid  matrix,  with  always  a  definite  space  between  them. 
In  general  the  cells  are  elongated  or  spindle-shaped,  with  a  delicate, 
hair-like  process  attached  to  either  extremity.  In  the  pulp-chamber  the 
cells  vary  somewhat  in  outline,  in  some  instances  being  spheroid,  in 
others  appearing  as  slender  filaments,  so  that  the  cell  proper  can  scarcely 
be  distinguished  from  its  processes.     A  third  class  of  cells  may  be  met 

*  The  pulp  not  only  occupies  the  central  cavity  in  the  tooth-crown,  but  the  canals  of  the 
roots  as  well;  therefore  the  form  of  the  pulp  corresponds  to  the  outline  of  the  pulp-cavity, 
already  described. 


THE    DENTAL    PULP 


361 


with,  from  which  three  or  more  filaments  are  given  off.  As  stated,  the 
distribution  of  cells  varies  considerably  in  different  parts  of  the  pulp, 
this  being  true  not  only  as  regards  numbers,  but  also  as  to  the  relations 
existing  between  the  cells.  In  the  coronal  portion  of  the  pulp  the  posi- 
tion assumed  by  each  individual  cell  appears  to  be  without  regard  to 
the  position  of  neighboring  cells,  while  in  that  portion  of  the  pulp  oc- 
cupying the  rootLcanals  the  cells  are  arranged  parallel  with  the  length 


Fig.  264. — Transverse  Section  through  Pulp.     Blood-vessels  and  Nerves  in  Cross  Section, 


of  the  root.  The  cells  are  least  in  number  in  the  interior  of  the  pulp, 
but  gradually  become  more  plentiful  as  the  periphery  is  approached.  (See 
Cells  of  Dentin  Papilla,  page  408). 

The  Odontoblasts  (Fig.  265). — The  most  active  cells  of  the  pulp  are 
those  directl}  on  its  periphery,  in  contact  with  the  dentin,  and  known 
as  the  odontoblasts.  The  odontoblastic  layer,  otherwise  known  as  the 
membrana  eboris,  is  composed  of  a  single  row  of  cells,  each  of  which 
contains,  near  the  extremity  most  distant  from  the  dentin,  a  well-defined 
nucleus.     They  are  large,  elongated  cells,  each  furnished  with  three  sets 


362 


HISTOLOGY 


of  fibers  or  processes — the  dentinal  process,  the  pulpal  process,  and  the 
lateral  process.  The  dentinal  process  or  processes — there  may  be  more 
than  one  present — communicate  with  the  deeper-lying  cells  of  the  pulp, 
while  by  means  of  the  lateral  processes  the  cells  are  brought  into  com- 
munication with  neighboring  cells.  The  processes  given  off  in  the 
direction  of  the  dentin,  or  the  dentinal  processes,  may  be  one  for  each  cell, 
in  which  case  they  are  of  considerable  size,  and  are  inclined  to  taper  as 


Dentin 


Blood- 
vessels 


Fig.   265. — Pulp  and  Dentin  in  Longitudinal  Section. 

they  enter  the  substance  of  the  dentin.  Again,  a  single  cell  may  give  off 
a  number  of  smaller  processes  in  this  direction.  The  odontoblasts  vary 
much  in  form  according  to  their  functional  activity.  Before  the  period 
of  dentinification  they  are  spheroid  or  pyriform,  during  the  period  of 
calcification  the  dentin  extremity  becomes  somewhat  flattened  and 
square,  while  in  advanced  years  they  again  return  to  their  primitive, 
rounded  form.  Covering  the  entire  surface  of  the  pulp  like  an  epithelium, 
the  odontoblasts  are  especially  closely  associated  at  the  end  nearest  the 
dentin,  forming  an  unbroken  layer,  while  the  pulpal  extremities  are 
inclined  to  assert  their  individuality  by  disassociation. 


THE    DENTAL    PULP 


363 


Blood-vessels  of  the  Pulp. — The  pulp  is  richly  supplied  with  blood- 
vessels, forming  networks  extending  principally  in  a  direction  parallel  to 
the  long  axis  of  the  tooth,  and  finally  terminate  in  a  capillary  plexus 
closely  associated  with  the  odontoblastic  layer.  The  veins  of  the  pulp 
are.  ordinarily  somewhat  larger  than  the  arteries,  and  form  numerous 
anastomoses.     This  organ  appears  to  be  destitute    of  lymphatics — at 


Fig.  266. — Section  through  Pulp.     Fig.  265  in  Transverse  Section. 

least,  none  are  known  to  occur  in  its  substance.  The  blood-vessels  of  the 
pulp  are  provided  with  a  longitudinal  layer  of  thinly  distributed  muscular 
fiber,  but  otherwise  the  walls  of  the  vessels  are  noted  for  their  delicacy. 
Nerves  of  the  Pulp. — After  entering  the  apical  foramen  either  by  one 
large  trunk  or  by  two  or  more  minute  ones,  the  fibers  pursue  a  parallel 
course,  breaking  up  but  little  or  giving  off  but  few  fibers  in  that  portion 
of  the  pulp  confined  to  the  canal.  When  the  expanded  or  coronal  portion 
of  the  pulp  is  reached,  numerous  subdivisions  occur  which  are  distributed 
in  every  direction,  and  ending  in  a  rich  plexus  beneath  the  odontoblastic 
layer,  or  membrana  eboris.     In  the  body  of  the  pulp  the  fibers  are  medul- 


3°4 


HISTOLOGY 


lated,  but  those  occupying  the  periphery  are  non-medullated  and  sup- 
posed to  pass  into  the  dentinal  tubes.  While  this  latter  hypothesis 
is  in  all  probabiltiy  correct,  such  a  distribution  of  the  germinal  fibers  has 
never  been  definitely  demonstrated.  Two  investigators  (Ball  and 
Magitot)  claim  to  have  partially  satisfied  themselves  in  regard  to  the  final 
distribution  of  the  non-medullated  fibers.  The  former  states  that  he  has 
traced  these  fibers  into  continuity  with  the  larger  medullated  fibers  in  the 
deeper  pulp-tissue,  and  claims  to  have  found  them  passing  through  the 


Main  Blood 
vessels 


Branching  of  Main 
Nerve-trunk  into 
Single  Fibers 


Branching  ofTMain 
Blood-vessels  into 
Capillaries 


Fig.  267 — Distribution  of  Blood-vessels  and  Nerves  to  the  Pulp  of  Human  Molar. 

{After  Gy si.)      X20. 


membrana  eboris,  beyond  which  point  they  assumed  a  direction  parallel 
to  the  dentinal  tubules.  This  theory  is  controverted  by  Magitot,  who 
claims  that  the  dentinal  fibers  are,  in  a  measure,  themselves  prolongations 
of  the  nerves,  being  so  constituted  through  the  medium  of  the  branched 
stellate  cells  which  lie  immediately  beneath  the  membrana  eboris,  and 
by  which  the  nerves  are  made  continuous. 

Nasmyth's  Membrane. — Nasmyth's  membrane,  otherwise  known 
as  the  enamel  cuticle  or  persistent  dentinal  capsule,  is  an  exceedingly  thin 
and  peculiarly  indestructible  structure,  entirely  covering  the  enamel. 
As  to  the  presence  of  this  membrane,  which  can  be  demonstrated  only  by 
chemic  detachment,  there  appears  no  doubt,  but  in  regard  to  its  origin 
and  definite  structure  much  difference  of  opinion  has  been  expressed. 


THE    DENTAL    PULP 


365 


By  some  writers  (Tomes  and  Magitot)  it  is  maintained  that  it  is  continuous 
with,  and  similar  in  structure  to,  the  cementum  covering  the  root,  being 
an  extension  of  the  outermost  layer  in  the  region  of  the  neck  of  the  tooth; 
and,  in  view  of  the  fact  that  lacunae  are  found  in  its  substance,  this  theory 
would  appear  to  be  correct.  On  the  other  hand,  it  is  considered  to  be  a 
product  of  the  epithelium  (Huxley  and  Koiliker)  and  in  no  manner  con- 
nected with  the  cementum.  In  the  opinion  of  the  author,  it  would  be 
difficult  to  understand  how  the  theory  advanced  by  Tomes  could  be 
accepted.     During  the  entire  period  of  saccular  development  the  crown 


Alveolar  Wall 


■ 
- 


Cementum 


AlveolodentE 
Membrane 


Blood-vessels  of 
Alveolodental  Mem- 
brane in  Trans- 
verse Section 


Dentin 


Fig.  268. — Transverse  Section  through  Root  of  Human  Incisor  and  Surrounding 
Alveolar  Wall,  with  Alveolodental  Membrane  Intervening.      X40. 


of  the  tooth  is  in  close  relationship  to  the  enamel  organ,  this  structure 
intervening  between  the  forming  enamel  and  the  wall  of  the  tooth-sac, 
from  which  the  cementum  is  developed.  It  would,  therefore,  appear 
that  this  membrane  is  generated  from  the  external  epithelial  layer  of  the 
enamel  organ  by  a  change  in  the  character  and  form  of  these  cells.  Sud- 
duth  attributes  its  formation  to  a  metamorphosis  of  the  ameloblastic 
layer,  the  prismatic  cells  assuming  a  horizontal  direction.  The  amelo- 
blasts  are  observed  to  be  prismatic  in  form  up  to  the  point  at  which  the 
enamel  prisms  are  yet  unfinished,  but  as  the  surface  is  approached  they 
are  observed  to  shorten  and  widen,  and  near  the  gum-margin  they  as- 
sume a  longitudinal  direction  instead  of  being  at  right  angles  to  the 


366 


HISTOLOGY 


body  of  the  crown.  Mrs.  Emily  Whitman  devoted  much  time  to  the 
study  of  the  development  of  mammalian  teeth,  and  appears  to  be  of  the 
opinion  that  the  cuticula  dentis  is  the  result  of  a  change  in  the  form  and 
character  of  the  enamel  cells,  this  metamorphosis  taking  place  either 
before  or  after  calcification  of  the  underlying  tooth-tissues.  Nasmyth's 
membrane  shows  many  characteristics  which  differ  from  those  of  the 
body  of  enamel  subjacent  to  it,  serving  as  an  indestructible,  highly 
polished  surface-capping  to  the  enamel  prisms.  The  indestructible 
nature  of  this  membrane  by  reagents  would  appear  to  indicate  that  in 


Fig.  269. — Section  through  Root  of  Tooth,  Alveolodental  Membrane,  and  Alveolus. 
a,  Alveolus;  b,  Blood-vessel;  c,  Alveolar  Portion;  d,  Dental  Portion;  e,  Cementum. 

structure  it  is  closely  akin  to  the  structure  lining  the  dentinal  tubules, 
the  lacunae,  etc. 

Alveolodental  Membrane  (Figs.  268  and  269). — As  a  general  descrip- 
tion of  this  membrane  has  already  been  given  in  Part  I,  it  alone  remains 
to  treat  of  its  histologic  character,  which  may  best  be  accomplished  by  first 
referring  to  the  duties  which  it  has  to  perform.  These  may  be  divided 
into  three  classes — functional,  physical,  and  sensory.  The  functional 
office  is  accomplished  through  its  cellular  elements — the  osteoblasts  and 
cementoblasts;  the  physical  office  is  performed  by  the  fibrous  elements, 
through  which  the  tooth  is  fixed  in  its  position;  and  the  sensory  office 


ALVEOLODENTAL    MEMBRANE  367 

through  the  abundance  of  nerves,  which  are  richly  distributed  to  all  parts 
of  the  membrane.  We,  therefore,  find  in  this  structure,  besides  con- 
nective tissue,  cells,  fibers,  nerves,  and  blood-vessels.  The  principal 
cells,  as  already  stated,  are  the  osteoblasts  and  cementoblasts,  but  there 
are  aso  present  fibroblasts  and  osteoclasts. 

The  osteoblasts,  which  are  instrumental  in  the  upbuilding  of  a  portion 
of  the  alveolar  walls,  are  found  lying  against  the  bone,  between  the 
principal  fibers.  These  cells  do  not  appear  to  be  evenly  distributed, 
being  numerous  and  crowded  together  in  some  parts,  while  others  will 


Epithelium. 


Alveolar 
Wall 


Fig.  2  70. — Transverse  Section  through  Root  of  Tooth,  Alveolodental  Membrane,  Thin 
Wall  of  Alveolus,  and  Gingival  Tissue. 

appear  to  be  almost  destitute  of  them.  They  are  most  plentiful  in  the 
young  subject,  and  seldom  present  at  all  in  old  age.  In  youth  the 
alveolodental  membrane  is  thickest,  and,  as  the  building  of  bone  occurs 
on  the  inner  wall  of  the  alveolus,  it  can  only  progress  as  the  membrane 
becomes  reduced  in  thickness.  The  osteoblasts  are  polygonal  cells,  in- 
clining to  the  oval  form,  and  vary  greatly  in  size,  with  their  longest 
diameter  at  right  angles  with  the  surface  of  the  forming  bone.  During 
the  period  of  the  development  of  the  young  alveolar  wall  they  are  inclined 
to  be  crowded  together,  and  are  frequently  much  distorted  from  pressure 


368  HISTOLOGY 

upon  one  another.     As  age  advances  this  condition  becomes  less  pro- 
nounced, and  the  cells  separate  into  groups. 

The  Cementoblasts. — Stationed  upon  the  opposite  side  of  the  mem- 
brane, or  that  in  contact  with  the  root  of  the  tooth,  are  another  class  of 
cells — the  cementoblasts — or  those  cells  which  are  concerned  in  the 
formation  of  the  cementum.  Like  the  osteoblasts,  these  cells  are  found 
lying  between  the  principal  fibers  of  the  root-membrane.  They  differ 
in  form  from  the  osteoblasts,  notwithstanding  that  they  have  a  similar 
function.     Instead  of  the  polygonal  form  common  to  the  osteoblasts,  we 


Dentin 


Fig.  271. — Longitudinal  Section  through  Root  of  Growing  Tooth  near  the  Cervix. 

find  these  cells  to  be  more  or  less  flattened,  with  outlines  somewhat 
irregular.  Extending  from  the  body  of  the  cell,  which  contains  a  well- 
defined  nucleus,  are  a  number  of  irregular  processes,  which  penetrate 
the  neighboring  fibers  or  the  interfibrous  substance.  Unlike  the  osteo- 
blasts, the  cementoblasts  appear  at  all  times  to  be  evenly  distributed 
over  the  surface  of  the  cementum,  occupying  all  the  space  except  that 
taken  up  by  the  fibers  as  they  leave  the  cementum.  As  to  the  develop- 
ment of  the  osteoblasts  and  cementoblasts,  they  appear  to  be  carried  to 
the  fibrous  meshes  of  the  membrane  by  the  blood  as  leukocytes  or  ameboid 
cells,  after  which,  by  differentiation,  they  become  fitted  for  the  develop- 
ment of  bone  or  cementum,  assuming  their  respective  places  against 
the  surface  of  one  or  the  other  of  these  structures. 


ALVEOLODENTAL    MEMBRANE  369 

Fibroblasts  and  Osteoclasts. — Fibroblasts  and  osteoclasts  are  also 
present  in  the  alveolodental  membrane,  the  former  for  the  purpose  of  the 
increase  or  renewal  of  the  fibrous  tissue,  the  latter  being  functionally 
concerned  in  the  removal  of  a  part  of  the  alveolar  waUs  to  accommodate 
the  ever- varying  position  of  the  teeth,  or  acting  in  a  similar  manner  upon 
the  cementum  of  the  root.  The  osteoclasts,  or  giant-cells,  are  generally 
inclined  to  the  round  or  oblong  form,  and  usually  contain  a  number  of 
nucleoli.  They  vary  much  in  size,  and  are  seldom  branched  or  provided 
with  processes.     In  addition  to  the  four  classes  of  cells  already  mentioned 


Cementum 


Blood-vessel' 


Alveolodental 
Membrane 


Gingivae, 


Fig.  272. — Longitudinal  Section  through  Alveolodental  Membrane,  Gingival  Tissue, 
and  Root  of  Tooth.     Cervical  District. 

as  being  present  within  the  meshes  of  the  fibrous  tissue  of  the  root-mem- 
brane, there  is  another  class,  present,  however,  during  youth  only,  which 
appears  to  be  in  course  of  development,  and,  therefore,  without  apparent 
function. 

The  Fibers  oj  the  Alveolodental  Membrane. — The  principal  fibers  of 
the  alveolodental  membrane  are  those  which  extend  from  the  cementum 
on  one  side  to  the  alveolar  wall  on  the  other,  and  become  firmly  fixed 
at  either  extremity  by  penetrating  the  calcified  structures.  The  fibers 
are  all  of  the  white,  or  inelastic,  connective-tissue  variety.  It  is  by  means 
of  the  connective-tissue  fibers  that  the  actual  attachment  of  the  membrane 
24 


37° 


HISTOLOGY 


both  to  the  bone  and  to  the  cementum  takes  place,  the  fibers  passing 
directly  into  the  hard  tissues,  which  they  traverse  for  some  distance, 
being  here  known  as  Sharpeys  fibers. 

The  arrangement  of  the  fibers  is  somewhat  different  over  the  various 
parts  of  the  root.  In  the  region  of  the  gingival  margin  they  pass  out 
from  the  substance  of  the  cementum,  retaining  their  solid  form  or  dividing 
into  fasciculi  of  finer  fibers.     In  general  the  fibers  lie  parallel  with  one 


Fibers 


Fig.  273. — Section  showing  Fibers  of  Alveolodental  Membrane,  Attached  to' and  Passing 

Out  from  the  Cementum. 


another,  deviating  only  to  give  place  to  blood-vessels  and  nerves.  There 
is  some  variation  in  the  distribution  of  the  fibers  about  the  different 
gingival  surfaces.  Upon  the  labial  and  lingual  surfaces  they  pass  out 
directly  into  the  fibrous  tissue  of  the  gum,  and  soon  become  lost  in  this 
tissue.  On  the  mesial  and  distal  surfaces  the  fibers  passing  the  lower 
margin  of  the  alveolar  wall  join  the  fibers  of  the  neighboring  tooth. 
This  disposition  for  the  fibers  to  bend  toward  the  adjacent  tooth  is  first 
observed  at  the  various  angles  of  the  gingival  margin.     All  about  the 


ALVEOLODENTAL    MEMBRANE 


371 


free  border  of  the  gum  the  fibers  from  the  alveolodental  membrane 
assist  in  forming  this  tissue,  which  is  covered  by  a  dense  epithelial  coating 
of  moderate  thickness,  surrounded  or  surmounted  by  the  peridental 
fibers.  As  the  border  of  the  alveolar  wall  is  approached,  the  fibers  are 
observed  to  pass  under  the  proper  tissues  of  the  gum,  and  unite  with  the 
outer  periosteal  layer  overlying  the  outer  alveolar  wall.  The  fibers 
immediately  within  the  alveolus  are  slightly  inclined  in  an  apical  direction, 
while  those  occupying  the  central  portion  of  the  membrane,  or  that 
midway  between  the  apex  and  the  gingiva,  pass  nearly  straight  across 


Alveolus 


Cementum 


Pulp 


Dentin 


Fig.  274. — Transverse  Section  through  Growing  Roots  and  Alveolus. 

from  the  cementum  to  the  bone.  It  is  in  this  locality  that  the  largest 
and  strongest  fibers  are  found.  As  the  apex  of  the  root  is  approached 
the  inclination  of  the  fibers  is  crownward  from  the  cementum  to  the 
alveolar  wall.  In  this  situation  the  single  fibers  are  inclined  to  break  up 
the  fasciculi.  Immediately  surrounding  the  apex  of  the  root  the  fibers 
are  irregular  during  youth,  but  are  disposed  more  regularly  or  fan-like 
in  older  subjects. 

While  this  account  briefly  furnishes  a  description  of  the  distribution 
of  the  fibers  in  various  locations,  and  is  in  most  instances  correct,  they 
occasionally  vary  from  this  arrangement.  While  in  most  respects  the 
fibers  of  this  membrane  closely  resemble  the  corresponding  fibers  of 


372  HISTOLOGY 

attached  periosteum,  they  possess  some  peculiarities.  It  might  be 
supposed  that  the  fibers  passing  out  from  the  cementum  would  in  some 
way  differ  from  those  springing  from  the  alveolar  wall,  but,  with  the 
exception  of  being  somewhat  less  in  size,  they  are  otherwise  of  the  same 
character. 

Interfibrous  Elements. — Besides  the  various  forms  of  cells,  blood- 
vessels, and  nerves,  there  is  present  in  the  alveolodental  membrane  an 
interfibrous  tissue.  This  tissue  is  principally  composed  of  the  fibroblasts 
belonging  to  the  principal  fibers,  and  other  fibroblasts  accompanied  by 
delicate  fibers  which  appear  to  be  independently  distributed.  This 
interfibrous  tissue,  which  is  thus  seen  to  be  ordinary  fibrous  connective 
tissue,  appears  to  pervade  the  entire  membrane  wherever  sufficient  space 
is  found  to  permit,  of  its  presence.  In  some  parts  of  the  membrane  this 
tissue  appears  to  be  more  plentiful  than  the  principal  fibers  themselves. 
The  interfibrous  tissue  also  forms  an  investment  for  the  blood-vessels 
and  nerves  in  addition  to  the  tissues  properly  belonging  to  their  walls. 


373 


Fig.  275. — Evolution  of  the  Face.     (After  Haeckel.) 


CHAPTER  VIII. 


Embryology  of  the  Mouth  and  Teeth. 

The  nourishment  required  for  the  growth  of  the  embryo  and  its 
maintainance  during  the  earliest  part  of  its  development,  and  in  higher 
animals  during  the  whole  of  this  period,  is  supplied  either  from  the 
mother  by  means  of  a  placenta,  or  it  is  drawn  from  the  supply  of  concen- 
trated food-material,  stored  up  for  that  purpose  in  the  form  of  yolk  in 
the  egg.  The  formation  of  the  future  digestive  organs  of  the  mature 
organism  begins,  however,  already  at  a  very  early  stage  of  the  development 
of  the  embryo.  The  essential  epithelial  parts  of  the  digestive  canal 
are  derived  from  the  entoderm,  while  the  more  auxiliary  parts,  such  as 

muscles,  connective  tissue,  blood-vessels,  etc., 
which  are  not  present  in  the  lower  animals, 
but  become  more  and  more  conspicuous  as  we 
advance  in  the  scale  of  animal  organization, 
are  derived  from  the  visceral  layer  of  the 
mesoderm.  At  first,  the  primitive  gut  presents 
a  shallow,  wide  groove,  but  it  becomes  grad- 
ually deeper  by  a  folding  up  of  the  sides;  the 
edges  approximate  more  and  more,  and  when 
finally  a  union  has  taken  place,  there  is  a  tube 
formed — the  so-called  primitive  digestive  tract. 
While  the  latter  still  has  for  a  time  an  outlet 
in  the  form  of  the  vitelline  or  umbilical  duct, 
the  tube  is  closed  anteriorly,  as  well  as  pos- 
teriorly. As  development  proceeds,  there  becomes  noticeable  a  depression 
of  the  ectoderm  at  the  anterior  end  of  the  tube,  opposite  the  ventral  side  of 
the  latter,  and  here  ectoderm  and  entoderm  lay  closely  attached  to  each 
other,  forming  a  thin  membrane.  This  point  of  contact  of  the  two  layers 
is  known  as  the  oral  plate.  Soon  the  plate  ruptures,  and  thus  an  anterior 
aperture  of  the  gut,  the  oral  sinus  or  primitive  month,  is  formed. 

The  gradual  transformation  of  the  primitive  aperture  into  the  per- 
manent mouth  and  face  is  brought  about  mainly  by  the  development 
and  transformations  of  its  boundaries,  and  are,  in  brief,  the  following: 
About  the  end  of  the  third  week,  a  series  of  conspicuous  elevations  or 

374 


Fig.  276. — Diagram  showing 
the  relation  between  Ectoderm 
and  Entoderm  in  the  Mouth  of 
a  Mammalian  Embryo. 

a. I.  and  p.l.,  Anterior  and 
posterior  lobes  of  the  hyphysis 
m.  L,  medullary  tube;  ph., 
pharynx;  o.p.,  oral  plate;  x 
and  y,  ectoderm  which  pro- 
duces the  lip  and  teeth  of  the 
lower  and  the  upper  jaw,  re- 
spectively.    [Stbhr.) 


EMBRYOLOGY  OF  THE  MOUTH  AND  TEETH 


375 


processes  are  developed  around  the  opening,  from  the  front  and  the  sides. 
The  one  coming  down  from  the  head  is  called  fronto-nasal  process;  those 
coming  from  the  sides  are  arranged  in  form  of  parallel  bars  and  are 
known  as  arches.  There  are  five  pairs  of  arches;  in  fishes  they  give  rise 
to  the  formation  of  the  gills,  and  the  individual  pairs  are  separated  from 


gc 

Fig  277. 

Fig.  277. — Head  of  a  Young  Dogfish.     (Stohr.) 

Fig.  278. — Head  of  Human  Embryo  of  10  mm.     It  Illustrates  the  Phylogenetic  Rela- 
tion of  the  Visceral  Arches.     (Stohr.) 

g.c,  Gill  cleft;  m,  mouth;  n,  nasal  pit;  c.s.,  cervical  sinus;  g.c2,  second  gill  cleft;  h,  hyoid 
arch;  md,  mandibular  arch;  sp.,  (spiracle)  auditory  groove. 

one  another  by  slits.  In  human  embryos  there  is  no  communication 
between  the  inside  and  outside  of  the  arches,  but  there  are  distinct 
furrows  which  serve  as  lines  of  demarcation  between  the  individual 
bars.  On  account  of  these  formations  having  connection  genetically 
with  air-absorbing  viscera,  the  following  names  are  given:  Visceral  or 
branchial  arches,  inner  visceral  furrows  or  pouches,  external  visceral  fur- 
rows or  clefts.     From  the  accompanying  illustrations  it  can  be  clearly 


Fronto-nasal 

Process 
Lateral  Nasal 
Process 
Mesial  Nasal 
Process 
Maxillary 
Process 
Roof  of  the  Oro- 
pharynx 

Fig.  279.  Fig  280. 

Figs.  279  and  280  Present  the  Development  of  Various  Parts  of  the  Palate. 


Median  Portion 
of  the  Palate 


Dental  Ridge 


Lateral  Portion 
of  the  Palate 


{After  His.) 


seen  that  only  the  frontal  process  and  the  first  visceral  arch  participate 
in  the  formation  of  the  permanent  mouth  and  face;  the  other  four  arches 
give  rise  to  parts,  the  consideration  of  which  is  beyond  the  scope  of  this 
book.  In  regard  to  the  details  of  the  formation  of  the  individual  parts, 
tbe  following  statements  may  be  made: 


376 


HISTOLOGY 


The  Buccal  Cavity. — The  early  appearance  of  the  entrance  to  the 
alimentary  canal  is  found  in  the  formation  of  an  open  cavity  bounded  by 
the  primitive  maxillary  processes  above  and  mandibular  arch  below. 

The  cavity  thus  formed  is  the  common  buccal  space,  the  upper 
portion  being  the  respiratory  or  nasal  section,  while  below  is  the  true 
mouth.  The  cavity  of  the  mouth,  as  such,  does  not  exist  until  these  two 
are  completely  separated  by  the  palatal  plates  forming  the  future  roof 
of  the  mouth. 

Figure  281  shows  a  vertical  transverse  section  through  this  common 


Nasal  Cavity 


V  Nasal  Cartilage 


Buccal  [Cavity 


Dental  Ridge, 
Upper  Jaw 


s. 


Dental  Ridge, 
Lower  Jaw 


Tongue 


f       Dental .  Ridge, 
/  Lower  Jaw 


Fig.  281. — Vertical  Transverse  Section  through  Head  of  Human  Embryo,  about  the 

Tenth  Week.      X30. 

buccal  cavity.  At  this  early  period  the  lateral  walls  and  floor  of  the 
mouth  are  manifest  by  certain  cellular  elements,  but  the  roof  of  the 
cavity,  as  already  stated,  is  not  complete  until  the  palatal  plates,  now 
separated  by  the  tongue,  grow  inward  and  unite  at  the  median  line. 

The  Oral  Cavity.  The  Roof  of  the  Mouth. — When  these  two 
processes  which  arise  from  the  mesoblast  unite  at  the  median  line,  they 
establish  a  permanent  horizontal  septum,  dividing  this  part  of  the  stomo- 
deum  into  a  respiratory  or  nasal  section  and  an  oral  section,  the  mouth. 
The  cells  entering  into  this  part  of  the  fetal  head  at  this  time  (eighth  to 
tenth  week)  are  of  three  varieties,  being  connective-tissue  cells,  cartilage 


THE    FLOOR    OF    THE    MOUTH 


377 


cells,  and  epithelial  cells,  the  latter  being  distributed  in  a  layer  of  varying 
thickness  over  those  parts  destined  to  become  a  part  of  the  lining  mem- 
brane of  the  mouth. 

In  figure  282  (twelfth  week)  the  superior  maxillary  processes  are 
shown  united  and  the  permanent  separation  between  the  mouth  and  nasal 
cavity  established.  This  embryonal  bridge  is  for  the  most  part  made  up 
of  connective-tissue  cells,  about  isolated  bundles  of  which  osteoblasts 
arrange  themselves,  resulting  in  the  production  of  two  intermembranous 
bony  plates. 


Nasal  Cavity 


Dental  Ridge 


Tongue 


Meckel's  Cartilage 


r-\;- 


Cartilagenous  Septum 
of  Nose 


)  Dental  Ridge 

Oral  Cavity 


Dental  Ridge 


Anlage  of  Lower  Jaw 


Fig.  282. — Vertical  Transverse  Section  through  Head  of  Human  Embryo,  about  the 
Twelfth  Week,  showing  the  Single  Buccal  Cavity  Transformed  into  the  Oral  and  Nasal 
Cavities.     X30. 

By  the  fourteenth  week  a  further  advance  in  the  generation  of  the 
hard  palate  is  noted,  the  septum  now  being  largely  composed  of  calcified 
tissue.  The  disposition  for  these  primitive  bony  plates  to  exist  as  separate 
and  distinct  processes  is  exemplified  at  the  median  line  by  a  definite 
separation  formed  by  the  connective-tissue  sheath  from  which  they  are 
derived.  Covering  the  surface  of  the  hard  palate  there  now  appears  a 
thin  layer  of  mucous  membrane. 

The  Floor  of  the  Mouth.— Having  thus  briefly  noted  the  evolution 
of  the  roof  of  the  mouth,  let  us  next  consider  the  floor  of  the  cavity,  the 
tongue  and  its  attached  muscles,  together  with  considerable  glandular 


378 


HISTOLOGY 


tissue  making  up  the  bulk  of  this  district.  Figure  283  is  a  vertical 
transverse  section  through  the  floor  of  the  mouth  about  the  tenth  week 
in  the  human  fetus,  or  at  a  period  somewhat  later  than  that  shown  in  the 
previous  illustration.  An  examination  of  the  parts  in  general  at  a  time 
prior  to  this  is  of  little  value,  save  the  early  preparation  for  the  develop- 
ment of  the  teeth,  which  will  be  referred  to  later.     The  tissues  and  or- 


Fig.  283. — Section  through  Base  of  Tongue  and  Lower  Jaw.     X40. 

gans  here  shown  will  be  recognized  as  the  tongue  (.4),  the  glandular 
tissues  (B),  the  forming  jaw  (C),  with  developing  tooth-germs  at  D  D. 
The  tongue  appears  on  the  floor  of  the  mouth  between  the  thirtieth 
and  thirty-sixth  days  as  a  bud  from  the  mesoblast  covered  by  a  layer  of 
cells  of  epiblastic  origin.  The  muscle-fibers,  be  they  intrinsic  or  ex- 
trinsic, are  all  of  the  striated  variety.  In  a  very  short  time,  and  at  a 
comparatively  early  period,  the  tongue  becomes  an  independent  organ, 
presenting  most  of  the  characteristics  common  to  it  after  birth.  Not  a 
small  portion  of  the  floor  of  the  mouth  is  made  up  of  another  class  of 


THE    FLOOR    OF    THE    MOUTH 


379 


tissue  which,  although  eventually  a  distinct  organism,  is  composed  almost 
entirely  of  epithelium.  These  cells,  together  with  the  connective-tissue 
cells,  and  eventually  blood-vessels,  unite  in  the  production  of  a  true 
salivary  gland,  the  sublingual.  Figure  284  shows  the  early  character  of 
the.  tissue,  together  with  its  relation  to  surrounding  parts.  The  section 
is  one  from  the  region  of  the  premolars,  and  is  bounded  above  by  the 
tongue,  laterally  by  the  borders  of  the  jaw,  and  below  by  fibers  of  the 


Fig.  284.— Section  of  Sublingual  District.     X 100. 


mylohyoid  and  digastric  muscles.  Three  distinct  lobes  or  sections  of 
the  gland  are  observed,  the  two  largest  being  separated  by  a  reticular 
network  of  connective  tissue. 

The  general  character  of  these  developing  glands  even  at  this  early 
period  (about  the  twelfth  week)  appears  to  be  very  similar  to  the  matured 
organ,  being  composed  of  a  number  of  small  tubes  emptying  into  a  single 
duct,  constituting  a  gland  of  the  compound  tubular  variety. 

Let  us  next  give  some  consideration  to  the  embryology  of  the  mouth 


380  HISTOLOGY 

in  its  entirety;  and  to  do  this,  it  is  necessary  to  make  sections  of  the  parts 
in  various  directions. 

The  growth  of  the  cavity  is  usually  studied,  and  probably  to  the 
best  advantage,  by  vertical  transverse  sections,  and  attention  will  first  be 
called  to  a  number  of  sections  made  in  this  way,  beginning  at  the  lips  and 
passing  backward  through  the  incisor  region,  and  finally  through  the 
districts  occupied  respectively  by  the  cuspids  and  molars.     The  period 


Fig.  285. — Embryonal  Labial  Mucous  Membrane. 

at  which  such  an  investigation  is  made  has  much  to  do  with  the  character 
of  the  tissue  involved,  but  the  time  best  suited  to  the  purpose  is  included 
between  the  fortieth  and  sixtieth  days.  At  this  time  nearly  all  the  tissues 
making  up  the  organs  and  parts  which  enter  into  the  construction  of  the 
cavity  have  advanced  to  such  a  degree  of  perfection  that  the  investigation 
may  proceed  with  considerable  satisfaction. 

Figure  285  shows  a  cross-section  through  one  of  the  primitive  labial 
folds  about  the  period  named.     Little  is  to  be  observed  in  this  district 


THE    FLOOR    OF    THE    MOUTH 


38l 


at  this  early  period  except  the  simple  cells  of  three  varieties  which  serve 
to  make  up  the  parts,  but  attention  is  at  once  attracted  to  the  abundant 
thickness  of  the  epithelium  given  to  the  lip. 

If  a  section  be  made  somewhat  to  the  distal  of  that  previously  shown, 
a  marked  change  in  the  relationship  existing  between  the  various  cell 
layers  is  observed  in  a  body  of  cells  of  another  character,  those  which  are 


Fig.  286. — Section  through  Base  of  Jaw. 

destined  to  become  the  cartilage  of  Meckel,  and  about  which  the  younger 
layer  of  cells  of  mucous  membrane  are  observed  outlining  a  new  district. 
If  a  section  be  made  through  this  same  location,  say  about  the  forty- 
eighth  day,  a  vast  change  in  the  appearance  of  the  parts  is  noticed  (Fig. 
281).  The  buccal  walls  of  the  mouth  have  in  a  measure  become  com- 
plete by  a  union  of  the  upper  and  lower  sections,  the  union  at  this  time 
being  accomplished  through  the  agency  of  the  embryonal  epithelium. 
A  cartilaginous  nasal  septum  has  made  its  appearance,  and  active  prepa- 
ration for  the  ossification  of  the  maxillae  is  apparent.     In  the  center  of  the 


382 


HISTOLOGY 


section  is  a  distinct  body  of  cells  forming  Meckel's  cartilage,  and  early 
preparations  for  the  growth  of  the  teeth  may  be  seen  at  a  by  a  dipping 
down  of  the  surface  epithelium. 

A  transverse  section  through  the  same  district  about  the  sixtieth 
day  (Fig.  287)  shows  all  the  parts  strongly  differentiated.  Ossification 
has  taken  place  to  a  considerable  extent  in  the  lower  jaw,  the  two  halves 
being  at  this  period,  and  for  some  months  afterward,  separate  and  distinct. 


w 


■ ; 


# 


;*¥■  ■       * 


■ 


■  _ 


Fig.  287. — Section  through  the  Wall  of  the  Mouth  of  an  Embryo,  Sixtieth  Day. 


Many  muscle  bundles  are  observed  beneath  the  jaw,  and  beyond  these 
the  integument  with  its  numerous  blood-vessels  and  nerves,  most  of  which 
are  seen  in  transverse  section.  A  cross-section  upon  the  same  subject 
about  the  sixtieth  day  in  the  region  of  the  cuspids  finds  the  tissues  and 
organs  advanced  to  a  certain  degree  of  perfection.  (See  Fig.  283.)  The 
tooth-germs  of  the  cuspid  teeth  have  their  crowns  outlined  by  the  cells 
composing  them;  the  tongue  with  its  complex  muscular  arrangement 
has  become  a  specialized  and  independent  organ,  while  beneath  it  we  see 


THE    FLOOR    OF    THE    MOUTH 


3*3 


that  product  of  the  epiblast,  the  glandular  structure,  so  plentifully  supplied 
to  the  floor  of  the  mouth  jn  this  locality. 

Passing  further  back  into  the  region  of  the  molars,  the  appearance 
of  the  parts  does  not  differ  to  any  marked  degree  from  that  in  the  cuspid 
district,  except  in  the  general  distribution  of  the  muscular  fibers  of  the 


Fig.  288. — Longitudinal  Section  through  Chin  of  Embryo  Lamb. 


tongue,  and  the  appearance  of  the  submaxillary  gland,  here  appearing  in 
three  distinct  lobes  or  parts. 

It  has  been  previously  stated  that  sections  made  in  the  direction  of 
those  already  considered  are  usually  employed  to  study  these  parts,  but 
much  is  to  be  gained  by  supplementing  these  with  sections  made  in  other 
directions. 


384 


HISTOLOGY 


Figure  288  shows  a  longitudinal  section,  or  one  made  from  mesial  to 
distal  through  the  lower  jaw  at  or  near  the  median  line,  the  parts  included 
within  the  field  being  the  labial  folds  at  A,  the  mandible  at  C,  the  tongue 
at  D,  and  a  tooth-germ  at  E.  An  examination  of  the  lips  shows  them  to  be 
covered  with  a  varying  thickness  of  embryonal  epithelial  cells  which  are 
continued  backward  over  the  future  alveolar  ridge  and  thence  to  the 
hard  palate  above,  or  over  the  floor  of  the  mouth  and  the  surface  of  the 
tongue  below. 

Meckel's  Cartilage. — One  of  the  earliest  products  of  the  mesoblast 
is  that  which  results  in  the  production  of  Meckel's  cartilage,  which  is 


Fig.  289. — Diagram  to  Illustrate  the  Metamorphosis  of  the  Visceral 
Arches  during  Development. 
1,  Presents  the  first  visceral  arch,  it  gives  rise  to  Meckel's  cartilage,  which 
becomes  transformed  into  the  mandible.     {After  Wiedersheim.) 


closely  associated  with  the  growth  and  early  support  of  the  lower  jaw. 
In  the  beginning,  as  already  pointed  out,  the  mandibular  and  hyoid  arches 
resemble  one  another,  but  soon  after  they  become  fully  established  they 
take  on  different  functions,  and  with  this  become  dissimilar.  The  first 
appearance  of  this  cartilage  as  a  distinct  body  of  cells  is  found  about 
the  middle  of  the  second  month,  and  when  a  transverse  section  of  the 
jaw  is  made  for  the  purpose  of  studying  its  location  and  environments 
(see  Fig.  290),  it  is  found  near  the  base  of  the  fetal  head,  considerably 
below  and  to  the  outside  of  the  base  of  the  tongue.  At  mid-jaw  it  appears 
as  a  circular  body  of  cells  separated  from  the  surrounding  parts  by  a 
distinct  layer  of  elongated  cells.  Even  at  this  early  period  a  portion  of  the 
bony  structure  of  the  jaw  is  outlined  by  an  aggregation  of  connective- 
tissue  cells,  and  the  forming  cartilage  appears  to  subserve  the  purpose  of 


Meckel's  cartilage 


385 


controlling  the  outline  of  the  future  jaw.  The  bow-shape  of  the  cartilage 
is  manifest  as  We  pass  toward  the  symphysis  by  the  lateral  halves  approach- 
ing each  other  (Fig.  291),  but  the  circular  character  of  the  cartilage  in 
cross-section  is  still  retained. 

Figure  292  represents  a  section  through  the  symphysis  about  the 
eighteenth  week,  and  shows  the  two  halves  of  the  cartilage  closely  asso- 


Fig.  290. — Section  through  Base  of  Lower  Jaw,  showing  Meckel's  Cartilage. 

dated,  but  not  united,  the  separation  being  by  a  layer  of  connective- 
tissue  cells  passing  between  the  two.  It  will  be  noted  also  that  the 
cartilage,  instead  of  being  near  the  base  of  the  jaw  as  in  figure  291,  now 
appears  near  the  floor  of  the  mouth. 

Figure  293  shows  the  relations  existing  between  the  two  halves  of 
Meckel's  cartilage  and  the  growing  mandible  at  the  median  line  (a). 
25 


386 


HISTOLOGY 


It  also  illustrates  how  little  the  development  of  the  bone  is  dependent  upon 
the  cartilage,  the  growth  of  the  former  being  in  this  district  far  below 
and  apparently  distinct  from  the  latter.  Here,  as  in  the  upper  jaw,  the 
periosteal  cells  from  either  side  are  observed  to  unite  at  the  symphysis  and 
pass  as  a  somewhat  thickened  layer  between  the  two  bones,  the  only 
difference  in  the  final  change  which  takes  place  between  the  two  being 
that  in  the  upper  jaw  a  suture  results,  while  in  the  lower  jaw  a  layer  of 


Fig.  291. — Section  through  Lower  Jaw.     M.  C,  Meckel's  cartilage. 


solid  bone  is  formed.  The  character  of  this  cartilaginous  framework  as 
well  as  the  cells  which  divide  the  two  halves  is  shown  in  figure  294,  the 
cartilage  cells  being  oblong  or  cylindrical,  with  a  bountiful  supply  of 
intercellular  substance,  while  the  connective-tissue  cells  are  oblong  or 
spindle-shaped. 

As  soon  as  ossification  in  the  jaw  takes  place  to  any  extent,  the  car- 
tilage begins  to  atrophy,  that  portion  lying  next  to  the  jaw  degenerating 


MECKEL  S    CARTILAGE 


387 


Fig.  292. — Meckel's  Cartilage  (M.  C.)  at  the  Symphysis. 


388 


HISTOLOGY 


first,  so  that  by  the  tenth  or  twelfth  week  it  has  entirely  disappeared,  but 
before  this  takes  place  we  find  it  surrounded  by  the  periosteum,  and 
finally  completely  inclosed  within  the  bone. 

Figures  294  and  295  show  the  character  of  the  cartilage  cells  about  the 


Fig.  293. — Ossification  of  the  Mandible  at  the  Median  Line. 


time  that  they  are  beginning  to  atrophy.  It  will  be  observed  that  the  cells 
are  inclined  to  a  change  in  form,  and  that  they  are  proportionately  larger 
with  large  nuclei  and  nucleoli.  A  represents  the  district  nearest  the  jaw, 
and  the  cells  in  this  region  have  already  lost  their  characteristic  outline. 


Meckel's  cartilage 


389 


* 

-*'   "*"    «^-         ■**/*  ^\hWB 


Fig.  294. — Section  of  Meckel's  Cartilage  at  Median  Line. 


39° 


HISTOLOGY 


Fig.  295. — Cartilage  cells  in  the  beginning  of  atrophy. 


CHAPTER  IX. 

Development  of  the  Teeth. — The  Dental  Germs,  Enamel  Organ, 
and  Dentin  Organ;  the  Dental  Follicle;  Calcification  and 
Eruption. 

DEVELOPMENT  OF  THE  TEETH. 


Tooth  Band 


Stellate  Retic- 
ulum 


Ameloblasts 


Dentin  Papilla 


Fig.  296. — Developing  Tooth-germs.      X300. 

In  order  that  the  student  may  obtain  a  general  idea  of  the  structural 
changes  which  take  place  at  a  very  early  period,  and  which  eventually  result 
in  the  formation  of  the  teeth,  the  genesis  of  the  subject  will  be  briefly  re- 
ferred to.     Preparation  for  the  development  of  the  teeth  takes  place  as  early 

39i 


392  HISTOLOGY 

as  the  middle  of  the  second  fetal  month,  prior  to  the  formation  of  the  bony 
structures  which  finally  surround  and  give  support  to  the  organs.  At 
this  early  period  there  will  be  found  following  the  line  of  the  future  alveolar 
ridge  a  slight  heaping  up  of  the  surface  epithelium  {epithelial  ridge), 
while  immediately  beneath  this  proliferation  of  cells  there  appears  a  dip- 
ping in  of  the  deep  or  infant  epithelial  layer  (early  called  the  dental 
groove),  in  the  direction  of  the  future  alveolar  walls.  This  epithelial 
reflection  is  later  known  as  the  epithelial  band  or  tooth-band.  This  so- 
called  tooth-band  is  not,  as  might  be  supposed,  a  special  inflection  for 
each  tooth-germ,  but  is  continuous  from  one  distal  end  of  the  future  jaw 
to  the  other.  It  must  be  remembered  that  at  this  time  the  outline  of  the 
jaws  has  not  been  established,  and  the  tooth-band,  is  principally  instru- 
mental in  directing  the  location  of  the  dental  organs.  The  position  and 
form  of  this  epithelial  band  may  best  be  studied  in  vertical  transverse 
section.  When  first  making  its  appearance  it  is  somewhat  broad  and 
shallow  (dental  groove),  but  as  it  passes  more  deeply  into  the  embryonic 
tissue  it  partakes  of  the  shape  of  the  letter  V  with  its  open  end  directed 
toward  the  surface.  As  it  grows  in  length  the  free  extremity  of  the  band 
is  inclined  to  the  lingual,  its  external  surface  is  slightly  convex,  and  its 
internal  surface  correspondingly  concave.  After  the  tooth-band  has 
assumed  certain  proportions,  there  appears  on  its  inner  or  concave 
surface  a  thin  membranous  plate,  tooth  lamina  which  is  likewise  a  con- 
tinuous structure,  extending  the  full  length  of  the  epithelial  band. 

This  lamina  does  not  spring  from  the  free  margin  of  the  tooth-band, 
but  is  given  off  at  a  point  about  midway  between  this  border  and  the  base 
of  the  band.  The  structure  of  this  secondary  band  is  so  similar  to  that 
of  the  primary  one  that  it  should  be  considered  as  an  inflexion  from  it 
rather  than  a  new  structure.  We  find  them  between  the  seventh  and 
eighth  week,  the  maxillary  regions  giving  place  to  two  bow-shaped 
bands  (one  for  each  jaw),  each  of  which  is  preparing  to  throw  out  from 
its  secondary  lamina  ten  little  buds,  tooth  buds  which  soon  develop  into 
the  germs  for  the  twenty  deciduous  teeth.  When  these  buds  make  their 
appearance  they  are  simple,  rounded  bodies,  placed  somewhat  closely 
together,  but  they  do  not  long  retain  this  simple  form.  The  first  change 
which  takes  place  is  one  in  which  they  appear  to  lengthen  out  into  slender 
cords,  the  extremities  of  which  soon  begin  to  extend  laterally,  and  a  pear- 
shaped  enlargement  of  the  epithelial  cells  appears,  which  by  invagination 
later  assumes  a  bell-shaped  outline,  which  phenomenon  is  rapdly  in- 
creased by  a  proliferation  of  connective-tissue  cells  forcing  into  the 
concavity. 


DEVELOPMENT    OF    THE    TEETH 


393 


This  bell-shaped  proliferation  of  cells,  given  off  directly  from  the 
tooth-band,  to  which  it  continues  for  a  time  attached,  together  with  the 
specialized  connnective-tissue  cells  crowding  into  its  concavity,  constitute 
the  tooth-germs,  the  former  being  the  enamel  organ,  and  the  latter  the 
dentin  organ.  It  will,  therefore,  be  seen  that  the  enamel  is  dependent  upon 
the  oral  epithelium  for  its  development  (ecdermic),  while  the  dentin 
springs  from  an  entirely  different  source — the  connective  tissue  of  the  jaw 


Epithelium  of 
Tongue 


, 


Epithelium  of  Lower  Jaw 


';  I'/'    ...    Tooth  Band 


Infant  Layer  *&&' 
of  Cells  K"  ' 


Forming  Bone 


•  -  - 


V 


Stellate  Retic- 
ulum 


»    Ameloblasts 
Dentin  Papilla 


Fig.  297. — Developing  Tooth-germs,  Twelfth  Week.      X40. 


(endermic).  The  enamel  organ  rapidly  undergoes  a  cellular  transfor- 
mation: its  concavity  is  increased,  and  the  bell-shaped  outline  more 
strongly  defined.  Accompanying  this  change  in  form  it  gradually  recedes 
from  the  surface,  and  its  connection  with  the  tooth-band  becomes  less 
secure.  The  connective-tissue  cells,  which  have  been  rapidly  filling  in 
the  concavity  of  the  enamel  organ,  are  also  preparing  to  take  upon  them- 
selves a  special  function,  that  of  the  formation  of  the  dentin.  Up  to  this 
period  (tenth  week)  the  enamel  and  dentin  germs  are  not  definitely 
separated  from  the  surrounding  cellular  structure,  but  now  a  gradual 
transformation  takes  place,  whereby  the  tooth-germs  become  enveloped 
in  a  sac-like  covering — tooth-sac,  this  together  with  its  contents  forming 
the  tooth- follicle. 


394  HISTOLOGY 

Enamel  Organ  (Figs.  296,  297,  and  298). — This  portion  of  the 
tooth-germ,  as  previously  stated,  is  derived  from  the  concave  or  lingual 
surface  of  the  tooth-band,  which  in  turn  is  derived  from  the  surface 
epithelium.  From  the  free  extremity  of  its  slender  cord-like  attachment 
it  spreads  out  and  forms  a  hood-like  covering  to  the  dentin  germ.  The 
surface  of  the  organ  contiguous  to  the  dentin  germ,  or  dentin  papilla,  as 
it  is  frequently  called,  is  concave  in  the  direction  of  the  oral  surface, 
being  thickest  over  the  center  of  its  concavity,  thinning  down  as  its  per- 
iphery is  approached.  Externally,  the  enamel  organ  is  covered  by  an 
epithelial  layer,  which  is  reflected  upon  its  inner  surface  or  that  in  contact 
with  the  dentin  papilla.  These  two  layers  are  named  according  to  their 
location,  the  external  and  internal  epithelium  of  the  enamel  organ.  Placed 
between  these  two  layers,  and  constituting  the  bulk  of  the  organ,  are 
numerous  stellate  bodies  which  penetrate  a  layer  of  rounded  cells,  the 
stratum  intermedium,  and  finally  reach  the  internal  epithelial  layer 
known  as  the  enamel  cells  or  ameloblasts.  It  is  from  this  internal  layer 
of  epithelial  cells  that  the  enamel  is  calcified,  and  they  are,  therefore, 
the  essential  cells  of  the  enamel  organ.  In  the  fully  developed  enamel 
organ,  there  are  to  be  found,  therefore,  four  distinct  layers  of  cells,  the 
external  epithelium,  stellate  reticulum,  stratum  intermedium,  and  internal 
epithelium,  or  ameloblasts.  As  its  name  implies,  the  function  of  the 
enamel  organ  is  principally  that  of  enamel  calcification,  but  in  the  opinion 
of  many  writers  its  primary  activity  is  that  of  molding  the  tooth-form 
as  represented  by  the  dentin  papilla,  and  it  is  not  until  this  latter  organ 
has  assumed  the  form  and  extent  of  the  dentin  of  the  future  tooth-crown 
that  calcification  begins. 

The  life  of  the  enamel  organ  may  properly  be  considered  as  begin- 
ning when  the  bulbous  extremity  of  the  specialized  cells  given  off  from 
the  lingual  face  of  the  tooth-band  become  invaginated,  and  from  this  by 
a  rapid  proliferation  of  its  cells  it  passes  on  by  successive  stages  assum- 
ing the  various  forms  common  to  it.  This  proliferation  and  differentia- 
tion of  cells  continues  up  to  the  time  of  beginning  of  calcification,  but 
with  the  advent  of  this  phenomenon  certain  parts  of  the  organ  begin  to 
degenerate.  This  degeneration  may  or  may  not  be  classed  as  an  atrophy 
of  the  cells  interested,  but  the  fact  that  a  new  tissue  is  generating,  and 
gradually  occupying  the  space  previously  taken  up  by  the  formative  cells, 
calls  forth  a  demand  for  the  removal  of  the  latter  by  the  former.  The 
cells  which  first  undergo  this  change  are  those  of  the  internal  epithelium 
and  stratum  intermedium,  the  individuality  of  these  two  layers  evidently 
being  kept  up  by  migratory  cells  from  the  stellate  reticulum.     It  is  argued 


DEVELOPMENT    OF    THE    TEETH 


395 


by  some  writers  that  the  external  epithelium  begins  to  atrophy  at  this 
period;  by  others  this  change  is  not  recorded  until  the  enamel  cuticle  has 
been  deposited  to  effectually  seal  the  young  tissue  and  protect  it  until  well 
desiccated.  While  there  appears  to  be  a  decided  disposition  upon  the 
part  of  this  outer  layer  of  cells  to  change,  they  do  not  dissappear,  and  the 
alteration  is  not  one  which  affects  the  shape  of  the  cells,  for  they  remain 
flattened  or  prismatic  with  their  long  axis  parallel  with  the  anlage 
of  the  crown  and  most  likely  become  the  enamel  cuticle.     The  stellate 


FlG.  298. — Developing  Tooth-germs,  Enamel  Organ,  and  Dentin  Papilla. 

cells  making  up  the  bulk  of  the  organ  are,  in  common  with  those  which 
inclose  them,  continually  undergoing  a  degenerative  change,  at  least  this 
is  true  of  those  cells  closely  associated  with  the  stratum  intermedium,  for 
in  this  location  they  rapidly  proliferate,  shed  their  many  processes, 
and  gradually  take  on  the  characteristics  common  to  this  layer  of  which 
they  eventually  become  a  part.  A  careful  examination  of  no  less  than  one 
hundred  enamel  organs  in  all  stages  of  development,  and  by  section  cut 
transversely,  obliquely,  longitudinally,  etc.,  fully  justifies  the  statement 
that  the  real  life  of  the  enamel  organ  begins  as  previously  stated,  and 
continues  until  the  structural  arrangement  of  the  enamel  is  completed. 


!96 


HISTOLOGY 


The  question  of  form  in  the  enamel  organ — that  is,  its  external  epithe- 
lium— is  one  which  may  be  advantageously  used  in  the  consideration  of 
the  life  and  function  of  its  different  cell  layers.  It  has  been  said  that  the 
apparently  extravagant  area  taken  up  by  the  enamel  organ  subserves 
the  purpose  of  reserving  space  for  the  growing  tooth-crown,  but  there  are 
many  reasions  why  this  theory  cannot  be  accepted.  In  the  first  place, 
the  extent  of  the  organ  or  the  space  existing  between  the  dentin  papilla 
and  the  outer  enamel  epithelium  does  not  in  very  many  instances  corre- 


Fig.  299. — Developing  Tooth-germs.     Longitudinal  Sections  from  Buccal  to  Lingual. 

spond  to  the  bulb  of  enamel  when  this  tissue  is  completed  at  a  given  point. 
In  the  developed  tooth  we  find  the  enamel  thickest  over  the  cutting-edges 
of  the  anterior  teeth  and  about  the  summits  of  the  cusps  of  the  cuspidate 
teeth,  while  these  same  parts  are  represented  during  the  cellular  stage  of 
development  by  the  external  layer  of  cells  closely  associated  with  the 
surface  of  the  papilla. 

Again,  the  outline  of  the  tooth  is  definitely  represented  by  the  cells 
making  up  the  dentin  papilla  (Figs.  298  and  299),  but  the  surrounding 
epithelial  cells  are  characterized  by  an  unbroken  semicircular  margin 


DEVELOPMENT    OF    THE    TEETH 


397 


describing  the  extent  and  form  of  the  enamel  organ.  Exception  may  be 
taken  to  this  hypothesis  from  the  standpoint  of  generative  changes,  and 
these  in  a  great  measure  have  much  to  do  with  the  relative  outlines  as- 
sumed by  the  two  organs,  but  by  studying  very  many  sections  representing 
nearly  every  stage  of  the  process,  and  all  of  them  in  a  measure  showing 
the  same  characteristics,  nothing  but  a  definite  opinion  can  result. 

Of  the  many  changes  in  general  form  which  the  enamel  organ  under- 
goes, none  are  so  pronounced  and  positive  in  character  as   those   de- 


Fig.  300. — Developing  Tooth-germs.     Longitudinal  Section  from  Mesial  to  Distal. 


scribed  by  the  inner  tunic,  and  first  recorded  when  the  bulbous  end  of 
the  specialized  cell  becomes  invaginated  by  the  mesodermic  connective- 
tissue  cells  forcing  themselves  into  it.  This  is  an  alteration  which  is 
gradual  and  continuous  up  to  the  time  of  beginning  of  calcification,  and 
while  the  cells  forming  the  dentin  papilla  are  generally  accorded  the  power 
of  "pushing"  or  "forcing"  their  way  into  those  derived  from  the  epi- 
blast,  the  latter  has  always  been  recognized  as  having  a  controlling  influ- 
ence over  the  former.     In  this  connection  a  reasonable  doubt  presents 


398 


HISTOLOGY 


itself  covering  the  theory  so  long  accepted  that  the  early  function  of  the 
enamel  organ  is  one  which  in  a  measure  superintends  the  contouring  of 
the  tooth-crown  as  first  represented  in  the  dentin  papilla.  When  the 
character  of  the  two  embryonal  tissues  making  up  the  two  germs  is  com- 
pared, we  find  the  dentin  germ  possessing  all  the  characteristics  favor- 
able to  a  rapid  proliferation  of  its  cells  resulting  in  a  highly  vascular, 
compact  tissue.  On  the  other  hand,  the  bulk  of  the  enamel  organ  is  a 
gelatinous-like  mass,  one  that  would  readily  succumb  to  the  pressure 


Fig.  301. — Developing  Tooth-germs  in  Transverse  Secton. 
A,  Stellate  reticulum.     B,  Papilla.     C,  Cartilage  cellsi. 

exerted  by  the  active  connective-tissue  cells  within  its  borders.  When 
thus  considered,  the  evidence  is  almost  sufficiently  convincing  to  reverse 
the  generally  accepted  theory,  placing  the  general  form  of  the  enamel 
organ  as  subservient  to  the  dentin  papilla. 

Figures  298,  299,  and  300  illustrate  some  of  the  variations  common  to 
the  general  form  of  the  enamel  organ,  and  afford  a  good  idea  of  the  rela- 
tionship existing  between  the  enamel  organ  and  the  dentin  papilla,  in  teeth 
both  of  the  simple  and  complex  class.     These  were  taken  from  sections- 


DEVELOPMENT    OF    THE    TEETH 


399 


which  represent  a  period  just  prior  to  the  generation  of  the  ameloblasts 
and  odontoblasts,  at  which  time  the  external  and  internal  epithelial 
layers  of  the  enamel  organ  most  closely  resemble  one  another  in  general 
outline.  It  is  from  this  aspect  and  from  sections  cut  longitudinally  that 
most  of  the  information  given  by  the  older  writers  has  been  derived. 
Few  attemps  have  been  made  to  show  this  organ  in  sections  transverse  to 
the  long  axis  of  the  tooth.  In  figure  301  the  germs  of  two  teeth  are 
shown  by  a  section  made  in  this  direction.     One  striking  feature  here 


a 


Fig.  302. 
A,  Inner  tunic  of  enamel  organ.      B,  Cells  of  dentin  papilla. 

illustrated  is  the  relationship  existing  between  the  inner  and  the  outer 
tunic  of  the  enamel  organ,  and  attention  is  called  to  the  apparent  coales- 
cence of  these  two  layers  at  those  points  which  represent  the  mesial  and 
distal  surfaces  of  the  developing  crowns.  This  condition  is  apparently 
brought  about  by  the  cartilage  cells  forcing  the  peripheral  cells  of  the 
enamel  organ  into  direct  contact  with  the  inner  tunic  completely  obliterat- 
ing the  stellate  reticulum  in  these  localities.  As  a  result  of  this  lateral 
pressure  the  outer  epithelial  cells  representing  the  labial  and  lingual 


4-00 


HISTOLOGY 


surfaces  have  become  widely  separated,  but  with  no  perceptible  alter- 
ation in  the  character  of  the  cells  composing  the  stellate  reticulum.  The 
relationship  existing  between  the  tooth-germs  and  the  surrounding  parts 
is  one  that  will  continue  throughout  the  generation  of  the  organs,  and 
makes  questionable  the  theory  that  the  stellate  reticulum  performs 
the  function  of  reinforcing  or  providing  the  ameloblasts  with  nutrient  or 
calcific  materal.  If  these  same  germs  were  examined,  as  they  usually 
are,  in  longitudinal  section  (see  Figs.  298,  299,  and  300),  the  investigator 


Fig.  303. 
A,  Cells  of  Dentin  papilla.     B.  Elongated  cells  of  inner  tunic. 

would  at  once  arrive  at  the  conclusion  that  there  was  an  equal  distribu- 
tion of  the  stellate  cells  about  all  sides  of  the  dentin  papilla.  In  their 
very  early  life  they  apparently  establish  an  equal  bulk  about  all  sides  of 
the  dentin  germ,  but  with  the  preparation  for  the  growth  of  the  alveolar 
walls  they  may  assume  the  proportions  shown  in  figure  301.  At  a  period 
corresponding  to  the  complete  envelopment  of  the  dental  germs  by  the 
dental  follicle  the  development  of  the  buccal  and  lingual  walls  is  well 
under  way,  but  as  yet  no  provision  has  been  made  for  the  septa  between 


DEVELOPMENT    OF    THE    TEETH  4OI 

the  teeth,  and  it  is  undoubtedly  to  the  approach  of  this  latter  phenomenon 
that  a  definite  lateral  pressure  is  brought  to  bear  upon  the  approaching 
of  the  follicles. 

Now  let  us  pass  to  a  consideration  of  some  of  the  characteristics  of 
the  various  cell  layers  composing  the  enamel  organ.  These  are  desig- 
nated according  to  their  location,  so  far  as  three  of  the  four  layers  are 
concerned;  in  fact,  the  remaining  cells,  or  those  which  receive  their  name 
from  their  form,  can  scarcely  be  classified  as  a  distinct  layer,  these  stel- 
late cells  not  being  of  uniform  thickness  in  all  parts  of  the  organ.  The 
first  layer  of  cells,  or  those  making  up  the  inner  tunic,  will  be  traced  from 
their  primary  spherical  condition  to  their  final  generation  into  ameloblasts. 
Figure  302  shows  the  character  of  these  cells  at  a  very  early  period,  corre- 
sponding to  the  sixteenth  week  in  the  human  embryo.  They  are  for  the 
most  part  spherical  or  slightly  oblong  multinucleated  cells,  and  are  more 
or  less  closely  associated.  They  partake  very  much  of  the  nature  of  the 
connective-tissue  cells  surrounding  them,  being  differentiated  from  these 
principally  by  a  transparent  zone  not  unlike  the  specialized  matrix  im- 
mediately surrounding  cartilage  cells.  About  the  first  change  recorded 
in  these  ceils  (see  Fig.  303)  is  one  in  which  they  become  markedly  elon- 
gated or  cylindrical,  but  during  this  process  of  differentiation  some  of  the 
cells  apparently  recede,  while  others  advance  in  the  direction  of  the 
papilla,  lining  up  in  a  single  layer  to  become  the  early  enamel  cells,  the 
cells  which  have  been  thus  forced  to  the  rear  subsequently  developing 
into  ameloblasts  as  the  older  cells  atrophy.  At  this  period  the  stratum 
intermedium  also  asserts  itself  in  the  form  of  a  distinct  layer  of  rounded 
cells,  to  be  described  later  on.  When  first  observed,  these  cylindrical 
cells  are  devoid  of  processes,  but  are  provided  with  rounded  extremities, 
with  little  or  no  variation  between  the  end  directed  toward  the  papilla 
and  that  looking  in  the  opposite  direction.  This  form  is  one  which 
persists  in  all  of  the  cells  included  in  this  layer  until  a  definite  body  of 
cells  is  formed  contiguous  to  the  dentin  papilla,  these  latter  cells  becoming 
more  markedly  elongated  and  further  differentiated  by  the  addition  of 
processes,  while  the  remaining  cells,  or  those  nearest  the  stratum  inter- 
medium, continue  for  a  time  unchanged.  The  next  alteration  in  the 
character  of  the  inner  tunic  is  one  well  illustrated  in  figure  303,  in  which 
the  body  of  the  generating  ameloblastic  cells  rapidly  recede  from  the  surf  ace 
of  the  papilla,  while  the  elongating  processes  reach  out  to  this  latter  point, 
all  of  this  occurring  before  the  appearance  of  the  odontoblasts.  Soon 
after  this  latter  change  in  the  cells  of  this  layer  the  odontoblasts  are 
developed  and  form  the  periphery  of  the  dentinal  tissue.  All  of  these 
26 


402 


HISTOLOGY 


changes  are  of  course  first  recorded  about  the  free  extremity  of  the  tooth- 
crown,  becoming  less  noticeable  as  the  union  of  the  outer  and  inner  tunics 
is  approached. 

A  study  into  the  special  characteristics  of  the  fully  developed  amelo- 
blasts  shows  that  these  active  cells  are  the  result  of  a  gradual  change  in 
the  character  of  the  columnar  epithelia  common  to  both  the  external  and 
internal  epithelial  layers  in  the  primitive  enamel  organ. 


A,  Stellate  reticulum. 


Fig.  304. 

B,  Stratum  intermedium.     C,  Ameloblasts. 
E,  Calcined  dentin.     F,  Odontoblasts. 


D,  Forming  enamel. 


Next  in  importance  to  the  internal  epithelial  layer  are  those  closely 
associated  cells  making  up  the  stratum  intermedium  (Fig.  305).  Primarily 
oval  or  spheroidal  in  form,  we  find  these  cells  gradually  assuming  a 
columnar  outline  and  occupying  a  position  parallel  to  the  long  axis  of  the 
crown.  It  may  be  said  that  the  general  character  of  these  cells  is  inter- 
mediate between  those  destined  to  become  the  proper  enamel  cells  and 


DEVELOPMENT    OF    THE    TEETH 


403 


those  stellate  cells  making  up  the  bulk  of  the  organ.  There  appears  to 
be  much  confusion,  at  least  considerable  doubt,  in  regard  to  the  office  of 
the  cells  of  the  stratum  intermedium.  It  is  most  likely  that  these  cells  are 
not  only  intermediate  in  character,  but  are  also  intermediate  in  func- 
tion to  those  cells  upon  either  side  of  them,  recruiting  the  ameloblasts  as 
they  fall,  while  in  turn  they  themselves  are  supplied  with  nutriment  from 
the  enamel  pulp  or  stellate  reticulum.  No  stronger  proof  that  these 
cells  are  secondary  in  importance  to  the  ameloblasts  need  be  mentioned 


A,  Generating  ameloblasts. 


Fig.  305. 
B,  Rounded  cells  of  stratum  intermedium. 


C,  Stellate  reticulum. 


than  reference  to  the  fact  that  they  are  always  more  generously  supplied 
to  those  parts  about  to  undergo  calcification.  Nor  is  their  increase  in 
numbers  the  only  reason  for  believing  that  they  are  thus  employed,  for 
at  the  same  time  those  cells  most  closely  associated  with  the  developing 
ameloblasts  take  upon  themselves  a  decided  change  in  outline.  This 
alteration  may  be  brought  about  by  the  conditions  which  influence  the 
shapes  of  all  cells,  i.e.,  by  the  pressure  of  surrounding  cells  or  by  their 
preparation  for  functional  activity,  or  both.     Some  of  the  older  writers 


404  HISTOLOGY 

speak  of  the  cells  of  this  layer  as  being  branched,  and  in  this  way  closely 
resembling  those  of  the  stellate  reticulum.  By  strong  amplification 
it  is  somewhat  difficult  to  distinguish  between  the  two  layers,  but  most 
certainly  if  there  are  branched  cells  present  they  are  confined  to  the 
intermediate  zone,  and  should  properly  be  classed  with  those  of  the 
stellate  reticulum. 

The  cells  of  this  layer  do  not  long  remain  columnar  with  a  general 
direction  at  right  angles  to  the  forming  ameloblasts,  but  they  become 
spheroidal  and  extremely  closely  associated  in  the  deeper  portion  of 
the  layer;  in  fact,  cells  corresponding  to  these  in  general  appearance 
may  be  found  in  connection  with  the  fully  developed  ameloblasts,  being 
observed  to  best  advantage  by  the  aid  of  a  high  power  objective  and 
a  full  flood  of  light  from  a  powerful  sub-stage  condenser.  The  cells 
thus  found  appear  to  be  distributed  at  regular  intervals  about  the  amelo- 
blastic layer,  and  are  so  closely  allied  to  the  cells  of  the  stratum  inter- 
medium that  they  may  be  considered  as  migratory  cells  from  this  layer. 
In  the  earlier  stages  there  appears  to  be  no  definite  line  of  demarcation 
between  the  cells  of  the  inner  tunic,  and  those  composing  the  stratum 
intermedium,  but  soon  after  the  establishment  of  the  ameloblasts  the 
two  layers  are  strongly  differentiated  by  the  interposition  of  a  highly 
transparent  membrane  covering  the  outer  extremities  of  the  amelo- 
blasts. After  they  are  thus  definitely  separated  from  the  enamel-forming 
cells,  a  most  radical  change  takes  place  in  their  character;  they  become 
markedly  elongated,  and  by  anastomosing  form  a  series  of  continuous 
chain-like  belts  about  the  ameloblastic  layer,  the  number  and  further  char- 
acter of  which  are  dependent  upon  the  extent  to  which  the  ameloblasts 
have  performed  their  function.  If  at  any  time  there  is  a  similarity  between 
the  cells  of  the  stratum  intermedium  and  the  stellate  reticulum  it  is  at 
this  period,  because  the  former  cells  begin  to  lose  their  individuality, 
although  under  low  power  they  still  appear  as  a  distinct  layer  (Fig.  303). 

There  is  probably  no  body  of  cells  directly  interested  in  the  develop- 
ment of  the  tooth  tissues  so  widely  discussed  as  those  making  up  the  so- 
called  stellate  reticulum,  and  while  the  chief  basis  for  argument  has  been 
with  reference  to  their  function,  the  general  character  and  form  of  the 
cells  have  received  but  little  consideration.  Ever  since  the  first  description 
of  this  portion  of  the  enamel  organ,  the  cells  therein  have  been  charac- 
terized as  "star-shaped,"  and  while  this  stellate  form  is  the  most  common, 
it  is  by  no  means  a  universal  condition.  The  form  of  the  cells  in  common 
with  the  other  cells  composing  the  organ  appears  to  be  much  influenced  by 
the  position  which  they  occupy,  and  by  the  age  of  the  organ,  those  cells  in 


DEVELOPMENT   OF   THE    TEETH  405 

the  region  of  the  inner  tunic  partaking  of  the  globular  form  characteristic 
of  this  layer  (see  C,  Fig.  304),  while  those  closely  associated  with  the  outer 
tunic  are  inclined  to  be  columnar  or  somewhat  elongated.  While  the 
cells  in  these  respective  locations  are  more  or  less  influenced  by  their 
environments,  they  still  retain  to  a  certain  extent  the  stellate  feature 
by  their  many  processes.  But  it  is  in  the  center  of  this  myxomatous 
epithelial  product  that  the  most  perfect  stellate  cells  are  located.  We 
find,  therefore,  where  this  part  of  the  organ  is  of  the  greatest  width, 
that  the  true  stellate  cells  are  the  most  numerous,  while  at  the  summit  of 
the  crown  and  at  the  base  of  the  organ,  at  both  of  which  points  the  outer 
and  inner  epithelial  layers  are  closely  associated,  the  star-shaped  cells 
are  almost  wanting.  In  the  study  of  this  layer  very  much  depends  upon 
the  thinness  of  the  section,  only  the  thinnest  possible  sections  affording 
an  opportunity  for  a  correct  conception.  This  is,  of  course,  true  of  all 
parts  of  the  organ,  but  the  peculiar  character  of  the  stellate  reticulum 
makes  it  especially  necessary  that  great  care  be  bestowed  upon  the  prep- 
aration of  the  section.  In  transverse  section  the  cells  present  no  charac- 
teristic differences  from  those  shown  when  the  section  is  made  longitudi- 
nally. One  very  pronounced  feature  about  the  cells  of  the  stellate  retic- 
ulum is  the  granular  appearance  of  their  protoplasm,  resembling  very 
closely  the  flattened  squamous  cells  from  the  epithelium  of  the  mouth, 
and  it  is  no  doubt  this  special  feature  which  furnishes  the  ground  for  the 
opinion  of  many  writers  that  it  is  a  peculiarly  modified  epithelium.  One 
peculiarity  in  connection  with  this  tissue  which  is  contrary  to  the  generally 
accepted  character  of  epithelial  cells  is  the  abundance  of  intercellular 
cement  substance;  but  when  the  many  minute  spines  or  processes  are 
considered  as  a  part  of  the  individual  cell,  the  proportionate  quantity 
of  cellular  and  intercellular  substance  is  somewhat  decreased.  The 
connecting  processes  are  quite  similar  to  those  described  by  Stohr  as 
connecting  bridges  of  protoplasm,  while  the  cells  themselves  may  be 
otherwise  described  as  prickle-cells.  The  change  in  the  form  of  the  cells 
of  this  layer  is  not  due  to  the  presence  of  neighboring  cells,  as  in  the  case 
with  most  epithelial  ceils,  but,  being  soft  and  extremely  plastic,  it  is  more 
than  likely  that  their  form  is  strongly  influenced  by  the  tension  of  their 
connecting  filaments.  One  of  the  most  marked  alterations  in  the  general 
character  of  this  part  of  the  enamel  organ  is  that  which  takes  place 
at  a  time  corresponding  to  the  beginning  of  amelification,  and  is  no 
doubt  attributable  to  this  phenomena.  The  cells  which  up  to  this  period 
have  remained  widely  separated  now  become  more  closely  associated, 
not  so  much  by  a  change  of  position  as  by  what  appears  to  be  an  increase 


406  HISTOLOGY 

in  the  size  of  the  cell  body  with  a  corresponding  decrease  in  the  length 
of  the  anastomosing  processes. 

It  is  a  fact  admitted  by  most  histologists  that  the  peculiar  star-like 
nature  of  the  cells  of  the  stellate  reticulum  is  one  principally  brought 
about  by  postmortem  changes,  and  that  in  reality  they  are  polygonal  cells 
filling  up  a  greater  part  of  the  tissue  with  but  little  intercellular  sub- 
stance. That  some  shrinkage  and  distortion  does  take  place  may  be 
proved  by  the  examination  of  a  section  which  has  accidentally  or  other- 
wise become  for  a  moment  dry  during  its  preparation,  in  which  case 
little  can  be  seen  but  the  connecting  processes,  and  even  these  are  much 
shrunken.  All  the  cells  contained  within  the  organ  are  more  or  less 
affected  by  this  procedure,  but  none  of  them  exhibit  such  a  marked 
change  in  the  outline  as  those  of  the  stellate  reticulum. 

The  layer  of ,  cells  which  is  usually  considered  of  least  importance 
is  that  which  makes  up  the  outer  tunic.  In  the  young  enamel  organ 
the  cells  partake  very  much  of  the  nature  of  these  forming  the  inner 
tunic,  but  the  older  the  organ  becomes,  the  more  dissimilar  are  the  two 
layers.  Primarily  this  layer  is  constructed  of  a  single  row  of  elongated 
cells,  placed  with  remarkable  regularity,  upon  the  inner  side  of  which  are 
a  number  of  similarly  formed  cells  variously  disposed,  but  with  a  common 
direction  at  right  angles  to  those  previously  referred  to.  Like  the  inter- 
nal epithelial  layer,  the  cells  of  the  outer  tunic  partake  more  or  less  of 
the  nature  of  the  stellate  cells  in  passing  from  the  single  row  of  well- 
defined  cells  toward  the  stellate  reticulum.  While  in  the  beginning 
the  external  epithelial  layer  is  strongly  differentiated  from  the  sur- 
rounding cells,  this  is  of  but  short  duration.  The  atrophy  of  this  layer 
begins  with  the  appearance  of  the  fully  developed  ameloblasts,  by  which 
the  regular  arrangement  of  the  cells  is  greatly  disturbed  by  an  apparent 
breaking  up  of  the  entire  layer.  Many  reputable  writers  claim  that  the 
external  epithelial  layer  is  of  little  or  no  interest,  save,  as  Tomes  puts 
it,  "as  a  matter  of  controversy."  This  admission  upon  the  part  of 
so  eminent  an  authority  practically  opens  up  a  new  field  for  research, 
especially  so  when  we  consider  that  various  other  writers  (Waldeyer, 
Kolliger,  and  Magitot)  have  expressed  conflicting  opinions  in  regard 
to  it.  After  carefully  following  the  changes  which  occur  in  this  layer 
fom  its  earliest  inception  up  to  an  advanced  stage  of  calcification,  it 
would  appear  that  while  marked  changes  occur  in  the  character  of  the 
individual  cells  as  well  as  in  the  general  appearance  of  the  layer,  it  is 
nevertheless  persistive,  and  in  some  way  is  essential  to  the  process  of 
amelification  even  to  a  more  marked  degree  than  are  the  cells  of  the 


DEVELOPMENT    OF    THE    TEETH  407 

stellate  reticulum.  One  important  reason  for  this  belief  is  based  upon 
the  fact  that  in  the  cuspidate  teeth  there  appears  at  a  time  corresponding 
to  the  beginning  of  amelification,  a  decided  disposition  in  the  cells  of 
this  layer  to  dip  down  and  completely  divide  the  stellate  reticulum  be- 
tween the  forming  cusps.  That  this  alteration  is  one  instrumental  or 
essential  to  the  calcifying  process  receives  additional  proof  by  referring 
to  figure  298,  which  shows  the  fully  developed  enamel  organ  with  the 


Oral  Cavity- 


Tongue  n  l§13iSlk--  '  ' 


Fig.  306. — Section  through  the  Floor  of  the  Mouth  of  Human  Embryo. 
Twelfth  Week.       X30. 

exception  of  the  actual  appearance  of  the  ameloblasts,  and  the  lack 
of  any  attempt  upon  the  part  of  the  external  epithelial  layer  to  penetrate 
between  the  cusps. 

Dentin  Organ  (Fig.  297). — This  part  of  the  tooth-germ,  formed  from 
the  connective  tissue  of  the  primitive  jaw,  occupies  the  concavity  of  the 
enamel  organ,  and  at  an  early  period  begins  to  assume  the  form  of  the 
future  tooth-crown.  Thus,  primarily,  the  papillae  for  the  incisors  will 
have  their  cutting-edges  outlined  by  three  small  lobes,  each  of  which 
represents  a  separate  point  of  calcification,  while  the  papillae  for  the  molars 
will  be  molded  according  to  the  number  of  cusps  of  the  future  tooth,  a 
small  tubercle  making  its  appearance  for  each  cusp.  In  its  inception  the 
dentin  papilla  is  composed  of  cellular  tissue  identical  with  that  of  the  sur- 


408  HISTOLOGY 

rounding  parts.  The  growth  of  the  papilla  is  in  the  direction  of  the  sur- 
face; at  the  same  time  the  enamel  organ  forces  itself  more  deeply  into  the 
substance  of  the  parts,  not  only  overhanging  the  coronal  extremity  of  the 
papilla,  but  extending  about  and  inclosing  its  lateral  walls.  Accompany- 
ing the  growth  of  the  papilla  is  a  rapid  change  in  its  structure,  becoming 
more  vascular  throughout,  and  its  peripheral  cells,  differentiating,  form 
the  essential  dentin-forming  cells — the  odontoblasts.  This  layer  of  cells  in 
in  close  relation  to  the  enamel  cells  of  the  enamel  organ,  the  combined 
activity  of  the  two  finally  resulting  in  the  calcification  of  the  tooth-crown. 
The  dentin  papilla,  which  eventually  becomes  the  tooth-pulp,  decreases 
in  size  as  calcification  proceeds  in  the  dentin,  all  additions  to  the  calci- 
fying surface  taking  place  from  within;  while  the  enamel  organ  may  be 
said  to  increase  in  size,  the  calcific  action  in  the  enamel  progressing  from 
within  outward. 

The  Cells  of  the  Dentin  Papilla. — In  the  early  life  of  the  dentin 
germ,  the  cells  are  all  simple  embryonal  connective-tissue  cells.  After 
differentiation  takes  place  they  are  widely  scattered  and  are  of  four 
varieties;  spindle-shaped,  round,  stellate,  and  the  elongated  or  club- 
shaped  odontoblasts.  None  of  these  are  constant  in  location  except  the 
layer  of  odontoblasts,  which,  as  has  been  said,  are  arranged  in  a  single 
row  on  the  surface  or  the  periphery  of  the  organ,  this  zone  being  classed 
by  the  older  writers  as  the  membrana  eboris.  Like  the  ameloblasts  of 
the  enamel  organ,  the  odontoblasts  do  not  make  their  appearance  until 
the  papilla  has  assumed  certain  proportions,  this  about  corresponding 
to  the  size  and  form  of  the  dentin  of  the  future  tooth.  Immediately 
beneath  the  layer  of  odontoblasts  appears  a  zone  almost  devoid  of  cells. 
This  is  followed  by  a  district  in  which  the  cells  are  quite  numerous,  and 
finally  when  the  central  portion  of  the  papilla  is  reached  the  cells  are  again 
few  in  number  and  widely  scattered.  For  the  most  part  the  cells  in  the 
interior  of  the  papilla  are  spindle-shaped  or  stellate,  having  rounded 
nuclei  about  which  there  is  a  small  amount  of  protopalsm  which  pene- 
trates the  intercellular  substance  by  numerous  hair-like  processes. 

The  Odontoblasts. — These  are  club-shaped  or  flask-shaped  cells, 
each  provided  with  a  large  nucleus  which  usually  assumes  the  outline  of 
the  enlarged  end  of  the  cell  which  is  directed  toward  the  interior  of  the 
papilla.  From  the  opposite  end  of  the  cell,  or  that  directed  toward  the 
enamel  organ,  and  in  close  proximity  to  its  concave  surface,  one  or  more 
protoplasmic  processes  are  given  off.  These  persist  and  are  finally  encap- 
suled  within  the  calcified  dentin,  forming  the  dentinal  fibers.  These 
cells  are  very  closely  associated,  so  much  so,  in  fact,  that  their  enlarged 


DEVELOPMENT    OF    THE    TEETH  409 

extremities  are  almost  or  quite  in  actual  contact,  more  or  less  space  ex- 
isting between  the  constricted  portion  of  the  cells  as  they  pass  toward 
the  surface. 

Germs  for  Permanent  Molars. — Reference  has  been  made  to  the 
fact  that  the  enamel  organs  for  the  deciduous  teeth  are  given  off  from  the 
tooth-band  at  a  point  somewhat  distant  its  free  margin,  so  that  the 
tooth-band  is  continued  beyond  the  primitive  enamel  germ,  this  free 
margin  of  the  band  afterward  generating  the  enamel  organ  for  the  succeda- 
neous  tooth.  As  the  twelve  permanent  molars  are  not  succedaneous 
teeth,  some  other  means  must  be  provided  for  their  development. 

Opinions  of  various  writers  upon  this  subject  are  somewhat  conflict- 
ing. The  theory  is  advanced  by  some  that  as  the  jaw  increases  in  length 
the  tooth-band  and  lamina  primarily  provided  for  the  deciduous  teeth 
are  extended  backward,  first  giving  off  a  bud  for  the  first  permanent 
molar;  at  a  somewhat  later  period,  and  with  the  increase  in  the  growth 
of  the  jaw,  an  additional  bud  is  generated  for  the  second  molar,  the  third 
molar  being  provided  for  in  a  like  manner.  Another  theory,  and  one  gen- 
erally accepted  as  correct,  is  that  the  cords  for  the  permanent  molars  spring 
individually  and  directly  from  the  subepithelium.  There  may  be  found 
an  exception  to  this  in  the  case  of  the  first  permanent  molar,  which  some- 
times appears  to  have  its  origin  from  the  distal  follicular  wall  of- the  second 
deciduous  molar.  Whatever  theory  be  accepted  in  regard  to  the  genesis 
of  these  permanent  organs,  the  process  of  development  after  the  appear- 
ance of  the  primary  bulb  or  enamel  germ  is  identical  with  that  of  the  de- 
ciduous teeth. 

The  Dental  Follicle,  or  Tooth-sac. — During  the  early  life  of  the 
tooth-germ,  both  the  enamel  organ  and  the  dentin  papilla  are  differen- 
tiated from  the  surrounding  parts  by  dissimilarity  of  structure  only,  but 
as  development  proceeds,  a  more  definite  separation  appears  between  the 
tooth-generating  organs  ond  the  general  tissues  of  the  primitive  jaw, 
this  separating  medium  being  known  as  the  dental  follicle.  The  term 
''follicle"  is  only  one  of  a  number  applied  to  these  parts,  "dental  saccu- 
lus,"  " tooth-sac,"  and  other  appellations  being  employed  with  equal 
significance.  By  some  writers  it  is  customary  to  apply  the  term  " follicle" 
up  to  the  period  of  complete  closure,  the  term  "sac"  or  "sacculus" 
being  employed  after  that  time.  There  appears,  however,  to  be  little 
foundation  for  such  a  distinction,  the  terms  being  synonymous.  Again, 
the  follicle  is  frequent  referred  to  as  meaning  the  sac  and  its  contents, 
but  this  usage  is  a  misapplication  of  the  term.  There  appears  to  be  no 
well-founded  reason  why  the  follicular  wall  or  sac  should  not  be  referred 


4io 


HISTOLOGY 


to  as  such,  regardless  of  the  formative  organs  within.  As  to  the  develop- 
ment of  the  tooth-follicle,  it  appears  to  be  a  generally  accepted  theory 
that  at  a  very  early  period  there  is  developed  from  the  base  of  the  papilla, 
cells  which,  differentiating,  form  the  walls  of  the  follicle.  By  this  growth 
of  cells  the  periphery  of  the  papilla  is  first  surrounded,  and  this  step  is 
soon  followed  by  an  extension  of  the  cellular  structure  in  the  direction 
of  the  surface  epithelium,  to  the  deep  layer  of  which  the  cells  become 
firmly  attached,  and  in  so  doing  inclose  the  enamel  organ,  which  hangs 
like  a  hood  over  the  extremities  of  the  papilla.     The  tissue  thus  formed 


Wall  of  Follicle 


Calcified  Dentin 


Dentin  Papilla, 
or  Tooth. -pulp 


Wail  of  Follicle 


Developing  Bone 


FIG.  307. 


-Developing  Tooth  about  the  Fourth  Fetal  Month.     Appearance  of  the 
Tooth-follicle. 


from  the  base  of  the  dentin  germ  is  continuous  with  and  similar  in  its  origin 
to  the  pulp-substance.  The  primitive  tooth-germ,  during  the  formation  of 
the  follicular  wall,  is  found  swinging  in  a  membranous  pocket,  being 
supported  by  the  epithelial  band,  which,  in  turn,  is  attached  to  the  oral 
epithelium;  but  as  the  walls  increase  and  completely  inclose  the  germs, 
which  is  accomplished  about  the  fourth  fetal  month,  the  epithelial  band 
is  broken  and  the  second  or  saccular  stage  of  tooth-development  is  reached. 
The  walls  of  the  follicles  are  made  up  of  two  layers;  the  outer  layer  is 
dense  and  firm,  and  finally  becomes  the  dental  periosteum;  the  inner 
layer  is  thin,  frail,  and  in  the  recent  state  somewhat  transparent,  and  at 
an  advanced  period  assists  in  the  formation  of  the  cementum,  the  two 
layers  finally  evolving  into  the  alviolo-dental  membrana. 


DEVELOPMENT    OF    THE    TEETH 


411 


SECONDARY  OR  SACCULAR  STAGE  OF  TOOTH 
DEVELOPMENT. 

Having  thus  briefly  described  the  primary  stage  of  tooth-develop- 
ment, the  careful  study  of  which  can  only  be  pursued  with  the  aid  of  the 
microscope,  we  will  now  pass  to  the  secondary  or  saccular  stage.  By 
the  introduction  of  a  number  of  illustrations,  prepared  from  original 
dissections  by  the  author,  this  phase  of  the  subject  will  be  readily 
comprehended. 

Before  continuing  the  subject  of  tooth-development,  it  will  be  emi- 
nently proper  to  briefly  describe  those  parts  directly  concerned  in  the 
process.     At  a  very  early  period  of  fetal  life  we  find  preparations  are  being 


Calcified  Dentin 
Enamel 


Outer  Enamel 
Epithelium 


Epithelium  of 
Jaw 


Dental  Ridge 


Germ  for  Perma- 
nent Incisor 


Dental  Papilla 


Fig.  308. — Development  of  Deciduous  Incisor,  from  Human  Fetus. — 
{After  Gysi.) 

made  for  the  development  of  the  maxillary  bones.  That  these  are  about 
the  first  bones  to  be  called  into  functional  activity  accounts  in  a  measure 
for  their  very  early  development.  The  osteoblastic  activity  in  the  inter- 
cellular substance  destined  to  become  the  inferior  maxilla  begins  about 
the  middle  of  the  second  month  of  fetal  life,  while  at  a  somewhat  later 
period  a  similar  action  takes  place  in  the  region  of  the  superior  maxilla.  A 
detailed  description  of  the  body  of  these  bones  having  been  given  on  another 
page,  it  will  not  be  repeated  here,  but  to  that  portion  which  gives  lodg- 
ment to  the  tooth-germs,  and  which  in  a  measure  is  controlled  by  their 
presence,  some  attention  must  be  given,  and  for  this  purpose  the  mandi- 
ble will  principally  be  used. 


412 


HISTOLOGY 


Figure  309  represents  the  lingual  face  of  the  lower  jaw  after  removal 
from  a  three  months'  fetus.  Attached  to  it  is  the  remaining  portion  of 
Meckel's  cartilage,  which  by  this  time  is  much  wasted.  It  will  be  re- 
called that  this  cartilaginous  band  appears  in  the  mandibular  processes 
before  the  beginning  of  the  second  fetal  month,  being  formed  in  two 
distinct  halves,  the  free  ends  of  which  finally  unite  at  the  median  line, 
forming  a  continuous  support  or  framework,  about  which  ossification 
takes  place.  At  a  corresponding  period,  and  in  a  similar  manner,  two 
like  processes  are  thrown  out  for  the  superior  maxillae,  but,  unlike  Meckel's 
cartilage,  these  do  not  unite  at  the  median  line,  but  stop  short  of  this, 
the  space  thus  resulting  being  provided  for  by  two  additional  processes, 

That  Part  of  Cartilage  not  yet  Ossified. 


Head  of  Malleus 
Cartilage  of  Incus 

Handle  of  Malleus 


Ossified  Mandible 
Fig.  309. — Developing  Mandible,  Three-month  Fetus. 

which  shoot  down  from  the  region  of  the  forehead  and  provide  for  the 
development  of  the  intermaxillary  bones.  About  the  middle  of  the 
second  month  a  center  of  ossification  appears  in  the  neighborhood  of  the 
future  mental  foramen,  quickly  followed  by  others  at  the  symphysis  and 
at  the  angle  of  the  mandible.  These  secondary  centers  soon  unite  with 
the  primary  one,  and  by  the  end  of  the  second  fetal  month  the  osseous  con- 
tour of  the  primitive  jaw  is  established.  While  ossification  takes  place 
in  the  membrane  surrounding  Meckel's  cartilage,  the  cartilage  itself 
does  not  appear  to  be  directly  concerned  in  the  process,  and  by  the  sixth 
or  seventh  embryonic  month  the  mandibular  portion  completely  disap- 
pears, while  that  portion  near  the  tympanum  is  ossified  into  the  malleus. 
That  portion  of  the  bone  which  forms  above  Meckel's  cartilage  and  the 
inferior  dental  nerve  is  that  which  finally  gives  support  to  the  tooth-germs. 
This  cartilage  is  not  confined  to  the  human  species,  but  is  the  common 
heritage  of  reptiles,  rodents,  birds,  and  fishes,  in  all  of  which  it  gives 
support  to  the  developing  lower  jaw. 

Figure  310  represents  the  evolution  of  the  mandible  from  the  middle 


DEVELOPMENT    OF    THE    TEETH 


413 


> 


Fig.  310. — Evolution  of  the  Mandi- 
ble from  the  Third  Fetal  Month  to 
Birth,  Two-thirds  Actual  Size. 


of  the  third  fetal  month  to  the  time  of  birth.     It  will  be  observed  that 
during  this  interval  there  is  a  gradual  increase  in  the  size  of  the  bone, . 
but  little  alteration  in  its  contour.     By  a  constant  and  gradual  osseous 
deposit  about  the  distal  extremity  of 
the  bone  its  length  is  increased  to  accom- 
modate   the    additional    teeth    as    they 
make    their    appearance.     While    the 
external  form  of  the  bone  shows  but 
slight  variation  during  this  period,  the 
internal  structure,  or  that  wherein  the 
tooth-germs  lie,  is  undergoing  a  com- 
plete transformation. 

Figure  311  is  illustrative  of  these 
changes;  beginning  with  a  simple 
groove,  or  gutter,  into  which  the  tooth- 
follicles  hang,  the  follicles  exerting  a 
controlling  influence  over  its  form. 
Next  comes  the  appearance  of  septa  between  the  anterior  follicles,  which 
at  this  period  are  somewhat  irregularly  placed  in  the  arch,  followed  in  a 
few  weeks  by  a  well-defined  partition  between  the  cuspids  and  molars, 
until  finally,  at  birth,  each  follicle  is  inclosed  in  its  individual  crypt,  with 

the  single  exception  of  the 
second  molar,  in  which  the 
distal  septum,  or  that  which  is 
to  separate  it  from  the  perma- 
nent first  molar,  has  not  yet 
made  its  appearance.  As  the 
tooth-follicles  increase  in  size, 
by  the  development  of  the 
teeth  within,  they  become 
more  perfectly  inclosed  in  the 
bony  vaults,  the  sides  of  the 
alveolar  walls  arching  over 
and  almost  completely  in- 
closing the  developing  teeth. 
Figure  312  shows  the  lower  jaw  of  a  seven-months-old  child  embody- 
ing the  condition  above  referred  to.  No  sooner  have  the  crypts  grown 
to  this  extent,  than  the  resorptive  action  produced  by,  or  provided  for, 
the  advancing  crowns  speedily  results  in  their  downfall,  to  be  again 
built  up  with  the  evolution  of  the  permanent  teeth. 


Fig.  311. — Evolution  of  the  Mandible  from  Third 
Fetal  Month  to  Third  Month  after  Birth. 


414 


HISTOLOGY 


About  the  first  visible  sign  of  preparation  for  the  development  of  the 
teeth,  other  than  that  made  apparent  by  dissection,  may  be  observed  as 
early  as  the  beginning  of  the  third  fetal  month,  when,  upon  opening  the 
cavity  of  the  mouth  and  looking  upon  the  palate,  a  well-defined  infold- 


Mucous  Membrane  and  Periosteum  Lifted.. Up 


FlG.  312. — Inferior  Maxilla  of  Seven-months-old  Child. 

ing  of  the  epithelial  eminence  will  be  seen.  In  figure  313  this  condition 
is  shown  by  a  dissection  through  the  oral  cavity  of  a  four  months'  fetus, 
the  outer  fold  being  that  of  the  cheeks  and  lips,  while  within  are  the  hard 
palate  and  primitive   alveolar  ridge.     The   mouth   at  this  period  has 


Labiodental  Space 


Labial  Fold 


i!k— - — —--"""" 

j^^lfin  *•*!*'     'fcS 

Mm 

•                   f. 

Bi 

. 

Primitive  Dental 
Groove,  so  called 


Fig.  313. — Section  Through  the  Mouth  of  Four-month  Fetus. 


passed  the  rudimentary  state,  the  transverse  plates  which  contribute  to 
the  formation  of  the  hard  palate  having  approached  each  other  until  the 
oral  and  nasal  cavities,  heretofore  existing  as  a  single  buccal  cavity, 
have  become  separate  and  distinct.     The  infolding  of  the  oral  epithelium, 


DEVELOPMENT    OF    THE    TEETH 


415 


Maxilla 


Oral  Mucous  Membrane 
Superior     Fetal     Maxilla, 


as  outlined  on  the  summit  of  the  primitive  alveolar  ridge — the  primitive 
dental  furrow  or  groove — marks  the  position  of  the  tooth-band,  from 
which  are  given  off  the  incipient  tooth-bulbs.  For  the  purpose  of  further 
investigation,  a  dissection  of  these  parts  was  made  and  the  maxillary 
bones  removed,  after  which  they  were  divested  of  their  fibrous  covering, 
including  the  periosteum.  That  portion  which  overlies  the  palatal 
processes  was  readily  lifted  in  one  sheet,  while  that  upon  the  facial  surface 
was  separated  at  the  median  line  and 
stripped  independently  of  the  other 
(Fig.  314).  The  removal  of  these 
tissues  is  readily  accomplished  until 
the  margins  of  the  partly  formed 
alveoli  are  reached.  Here  the  peri- 
osteum dips  down  into  the  various 
crypts,  and  serves  as  a  lining  mem- 
brane for  them,  and  probably  con- 
tributes fibers  to  the  outer  layer  of 
the  follicular  walls.     After  advancing 

thus   far.    the   detached   tissues   may 

Fig.     314. 
be  grasped,  and  by  Careful  manipula-     showing  manner  of  Dissection  to  Expose 

tion  the  tooth-follicles  containing  the    Tooth-sacs- 

formative  organs  removed  from  their  respective  vaults  and  turned  over 

for  examination. 

Figure  315  shows  the  result  of  such  a  dissection.  On  the  left,  the 
palatal  plates  and  alveolar  walls  of  the  divested  bones  may  be  observed; 
on  the  right,  is  the  fibrous  covering,  which  has  been  turned  completely 
over  after  removal  from  the  bones,  having  firmly  attached  to  it  the 
ten  tooth-follicles  for  the  deciduous  teeth.  It  this  dissection  be  made 
without  the  precaution  of  lifting  the  periosteum,  the  follicles  would  not 
cling  to  the  oral  membrane  with  sufficient  tenacity  to  permit  of  their 
ready  removal.  It  may  be  of  some  interest  to  note  that  at  this  early 
period  the  position  of  the  follicles  containing  the  germs  for  the  lateral 
incisors  is  that  which  the  tooth  is  forced  to  occupy  up  to  and  frequently 
beyond  the  eruptive  period,  being  crowded  within  the  tooth-line  by  the 
central  and  cuspid  follicles,  in  consequence  of  which  the  lateral  crypts 
are  thrown  well  into  the  palatal  plates. 

Figure  316  represents  the  sacs  broken  down,  exposing  to  view  the 
dentin  papillae,  or  those  structures  destined  to  become  the  tooth-pulps. 
The  position  which  these  occupy  in  the  illustration  is  exactly  the  reverse 
from  that  which  they  assume  when  in  position  in  the  follicle,  being  thus 


4i6 


HISTOLOGY 


reversed  that  a  better  idea  of  their  shape  may  be  obtained.  Prior  to  the 
twelfth  or  thirteenth  week  of  fetal  life  this  incipient  bulb  or  papilla  is 
without  definite  form;  but  by  the  latter  period  each  papilla  begins  to 
assume  the  contour  of  the  future  tooth-crown,  as  faintly  outlined  in  the 
illustration,   those  of  the  incisors  presenting  the  angular  form  of  the 

Tooth-follicles 


Fig.  315. 


Maxilla  Oral  Mucous  Membrane 

-Dissection  upon  Superior  Maxilla,  Fourth  Fetal  Month,  Exposing 
Tooth-follicles. 


future  cutting-edge,  the  cuspids  that  of  the  single  cone,  while  the  coronal 
extremities  of  the  molars  are  represented  by  outlines  corresponding  to 
the  future  cusps  and  marginal  ridges.  Besides  the  dentin  papilla,  there 
is  contained  within  the  follicular  walls  the  organ  which  later  on  is  pro- 
ductive of  the  enamel,  but  up  to  this  time  has  been  actively  engaged  in 


Dentin  Papillae 


Maxilla 


Oral  Mucous  Membrane,  Turned  Completely  Over 

Fig.  316. 

molding  the  tooth-form  as  outlined  by  the  papilla.  To  return  to  the  tooth- 
follicle,  the  dissection  in  this  instance  being  upon  the  lower  jaw  of  a 
four  months'  fetus.  Figure  317  shows  the  mandible  removed  and  the 
dissection  carried  to  the  point  at  which  the  follicles  may  be  lifted  from 
their  bony  encasements,  this  being  accomplished  by  an  incision  along 


DEVELOPMENT    OF    THE    TEETH 


417 


the  base  of  the  bone,  followed  by  a  stripping  of  the  membrane  first  from 
the  facial  and  then  from  the  lingual  side  of  the  bone.  When  these  two 
flaps  reach  the  margin  of  the  crypts,  they  are  firmly  grasped  and  the 
follicles  removed  from  their  sockets,  as  illustrated  in  figure  318. 

At  the  beginning  of  the  saccular  stage  of  development  the  form   of 

Oral  Mucous  Membrane  and  Periosteum  Dissected  from  Mandible 


Attachment    of    Follicle    to    Oral   Membrane  (Enlarged  One-third). 
Fig.  317. — Manner  of  Dissection  to  Expose  Tooth-sacs. 


the  future  tooth-crown  is  well  outlined  by  the  dentin  papilla,  which  in 
figure  319  is  brought  into  view  by  a  dissection  of  the  walls  of  the  follicles 
shown  in  figure  3 18  without  breaking  the  attachment  existing  between  the 
two.  As  in  the  case  of  the  follicle,  much  confusion  of  terms  in  regard  to 
this  structure  exists,  the  "dentin  bulb,"  the  "pulp,"  "dentin  germ,"  and 


Tooth-follicles 


Oral  Membrane 


Follicle  or  Perma- 
nent First  Molar 


Fig.  318. — Tooth-follicles  Removed  from  Mandible,  Fourth  Fetal  Month. 


the  "  papilla"  being  used  ad  libitum,  without  apparent  regard  for  the  struc- 
tural changes  which  are  continuously  affecting  the  organ.  It  will,  there- 
fore, be  proper  to  simplify  this  conglomeration  of  terms  by  a  classification 
appropriate  to  the  various  stages  of  development.  The  term  "dentin 
papilla"  will  best  describe  this  part  of  the  tooth-germ  up  to  the  time  of 

27 


4i8 


HISTOLOGY 


beginning  of  calcification,  subsequent  to  which  the  term  "pulp"  should 
be  employed.  As  previously  stated,  the  enamel  organ,  until  this  period, 
has  been  principally  devoting  its  energies  to  the  molding  of  the  tooth- 
form,  and  it  is  not  until  this  model,  as  represented  in  the  papilla,  is  com- 
plete that  the  process  of  calcification  begins.  About  the  fourth  fetal 
month  preparations  for  the  calcification  of  the  deciduous  teeth  are  begun 
by  the  development  of  the  odontoblastic  cells  for  the  dentin,  which  first 
made  their  appearance  on  the  periphery  of  the  dentin  papilla,  the  summits 
of  the  various  cusps  in  the  molars  and  the  future  cutting-edges  of  the 
incisors  being  first  affected.     This  phenomenon  is  soon  followed  by  the 


Space  Occupied  by    Oral  Mucous 
Enamel  Organ  Membrane 


Follicular  Wall 


Dentin  Papillae 
Prior  to  Beginning 
of  Calcification 


Fig.  319. — Tooth-follicles  shown  in  Figure  314  Opened. 


appearance  of  the  ameloblastic  cells  for  the  enamel,  which  establish 
themselves  in  the  internal  epithelial  layer  of  the  enamel  organ. 

Figure  320  is  prepared  from  a  dissection  made  in  a  manner  similar 
to  that  shown  in  figure  316,  but  at  a  period  about  a  month  later,  being  from 
the  superior  maxilla  of  a  five  months'  fetus.  The  dissection  shows 
the  extent  of  calcification  at  this  period,  which  process  also  defines  the 
position  of  the  odontoblastic  cells  upon  the  extremity  of  the  papillae.  In 
the  incisors  (one  of  which  was  lost  in  the  preparation  of  the  specimen) 
the  dentin  may  be  seen  capping  the  cutting-edges.  The  cuspids  in  this 
subject  have  not  yet  begun  to  calcify,  although  it  is  not  unusual  to  find 
the  cusp  of  this  tooth  receiving  its  lime-salts  at  this  early  period.  In  the 
molars  the  summits  of  the  various  cusps,  as  well  as  a  portion  of  the  various 
ridges  descending  therefrom,  are  undergoing  the  change  produced  by 
the  impregnation  of  the  lime-salts.  It  is  quite  probable  that  these  delicate 
caps  are  at  this  time  composed  of  dentin  alone,  the  calcoglobulin  which 


DEVELOPMENT    OF    THE    TEETH 


419 


precedes  the  enamel  calcification  forming  somewhat  later.  From  this 
time  forward  the  pulp  undergoes  a  gradual  transformation  as  to  size  and 
form,  and  there  is  likewise  a  change  in  its  cellular  construction  on  those 
parts  adjacent  to  the  calcific  action.     While  the  outline  of  the  pulp  is 


Calcified  Caps     Tooth- pulp 


Maxilla; 


Oral  Mucous 

Membrane, 

Turned  Over 


FlG.  320. 

gradually  changing,  its  original  form  is  permanently  recorded  upon  the 
periphery  of  the  dentin  cap,  which,  when  once  formed,  is  immutable,  all 
additions  taking  place  from  within. 

Figure  321  illustrates  the  result  of  a  dissection  upon  the  lower  jaw 
of  the  same  subject,  disclosing  practically  the  same  conditions,  with  the 

Calcified  Dentin  Oral  Mucous  Membrane 


Space  Occupied  by 
Enamel  Organ 


Dentin  Papillas 


Fig.  321.— Tooth-follicles  Opened,  Exposing  Dentin  Papilla;  and  Beginning  of  Calcification 

Fifth  Fetal  Month. 

exception  of  the  sac  containing  the  developing  cuspid,  which  was  found 
with  a  slightly  calcified  cap  of  dentin.  This  slight  variation  between  the 
development  of  the  upper  and  lower  teeth  is  one  that  is  present  in  nearly 
every  instance,  the  latter  being  somewhat  in  advance  of  the  former. 


420 


HISTOLOGY 


In  figure  322  the  tooth-pulps,  with  their  primitive  cappings  of  dentin, 
have  been  removed  from  the  follicles,  and  a  better  opportunity  of  study- 
ing the  relations  between  the  two  parts  is  presented.  By  the  conversion 
of  the  coronal  extremities  of  the  dentin  papilla  into  odontoblasts,  and 
their  active  calcification,  some  positive  union  between  the  two  parts 
might   be   expected.     On   the   contrary,   the   dentine   caps   are  readily 


Fie  322. 

removed,  leaving  the  pulp  beneath  without  the  slightest  rupture,  so  that 
we  find  calcification  is  not  a  secretory  or  excretory  metamorphosis,  but 
that  the  change  takes  place  within  the  substance  of  the  papilla  itself, 
whereby  it  is  altered  from  an  organic  to  an  inorganic  substance. 

The  next  dissection  was  one  upon  the  mandible  of  a  six  months' 
fetus.  Figure  210  shows  the  tooth-follicles  removed  from  the  partially 
formed  bony  crypts  in  which  they  have  been  incased.     At  an  early 


Tooth-sacs  of 
Deciduous  Teeth 


Lingual  Surface  of  Mandible 
FiG.  323. 

period  of  fetal  life,  and  at  a  time  prior  to  the  completion  of  the  tooth- 
follicles,  there  is  deposited  beneath  the  tooth-germs  a  thin  layer  of  bone, 
which  at  once  begins  to  assume  the  form  of  the  partially  developed  fol- 
licular walls.  As  the  growth  of  the  follicle  proceeds,  there  is  a  correspond- 
ing increase  in  the  osseous  deposit,  the  alveolar  walls  extending  about 
and  accommodating  themselves  to  the  membranous  sacs.     Thus  we  find 


DEVELOPMENT    OF    THE    TEETH 


421 


in  this  portion  of  the  maxillary  bones  a  feature  peculiar  to  itself— that 
of  a  continuous  transformation  from  its  earliest  inception  to  the  adult 
period,  first  developing  about  the  temporary  tooth-sacs  and  completely 
incasing  them,  which  is  speedily  followed  by  complete  resorption  of  the 
walls,  again  followed  by  a  rebuilding  during  the  evolution  of  the  perma- 
nent teeth,  and  again  swept  away  with  the  loss  of  these  organs.     Figure  324 

Follicular  Wall        Calcified   Caps        Oral   Mucous  Membrane 


Dentin  Papilla 
First  Perma- 
nent Molar 


Tooth-pulps 

FiG.  324 

illustrates  the  opposite  side  of  the  same  jaw,  with  its  outer  or  facial  plate 
removed,  together  with  the  intervening  septa.  The  follicles  are  opened, 
and  the  extent  of  calcification  at  this  period  (six  months)  made  apparent. 
The  pulps  and  calcified  caps  are  approximately  as  found  when  dissected, 
save  a  slight  settling  of  all  the  parts.  The  incisors  have  calcified  to  about 
one-third  their  full  coronal  length;  the  unicusped  contour  of  the  cuspid 
has  been  established,  as  shown  by  the  deposit  of  the  lime-salts  upon  its 


m  m  M  **u* 


Fio.  325. 

summit,  and  the  upbuilding  of  the  mesial  and  distal  cutting-edges.  The 
first  molar  has  about  completed  its  occlusal  surface,  and,  while  the  cusps 
of  the  second  molar  are  nearing  completion,  there  is  a  lack  of  union  in  the 
central  and  distal  fossae.  Immediately  posterior  to  the  second  deciduous 
molar,  the  sac  containing  the  formative  organs  for  the  first  permanent 
molar  is  shown  opened,  exposing  to  view  the  dental  papilla,  which  at 


422 


HISTOLOGY 


this  early  period  has  assumed  the  form  of  the  future  tooth-crown,  and 
calcification  is  about  to  begin.  Figure  325  illustrates  the  extent  of  calcifi- 
cation in  the  deciduous  teeth  at  the  sixth  fetal  month.  As  the  growth  of 
the  teeth  proceeds,  it  will  be  observed  that  the  angularity  which  originally 
accompanied  the  calcifying  caps  of  the  molars  is  gradually  disappearing, 
as  is  also  the  tritubercular  form  of  the  incisors  and  cuspids,  this  change 
being  brought  about  by  the  deposition  of  enamel  to  the  parts. 

Figures  326  and  327  show  the  calcified  caps  removed  from  the  pulps, 


9'  9  M 


m*> 


Fig.  326. 

and  so  arranged  that  both  their  external  and  internal  anatomy  may 
be  studied.  Prior  to  this  time  there  has  been  but  little  alteration  in 
the  form  of  the  dentin  pulp,  only  a  gradual  decrease  in  its  size  being 
noted;  but  now  we  find  it  being  divested  of  many  of  its  angles,  particularly 
those  which  orginally  served  as  a  basal  form  for  the  coronal  extremities 
of  the  future  tooth.  With  the  disappearance  of  these  the  concavity  within 
the  cap  is  slowly  assuming  the  form  of  the  future  pulp-chamber. 

That  a  better  understanding  of  the  saccular  stage  of  tooth-develop- 


Fig.  327. 


ment  might  be  had,  a  transverse  section  was  made  through  the  molar  folli- 
cles, as  shown  in  figure  328.  In  this  the  attachment  of  the  follicular  walls 
to  the  deep  epithelial  layer  is  visible,  while  within  the  walls  is  the  enamel 
organ,  the  calcified  dentin,  and  the  tooth-pulp.  As  previously  stated, 
the  enamel  organ  is  seen  suspended  above  and  forming  a  hood-like  invest- 
ment to  the  calcifying  structure.  This  organ  not  only  overhangs  the  occlu- 
sal surface  of  the  tooth-crown,  but  completely  envelops  the  sides  of  the 
calcified  cap  and  dentin  pulp.     Previous  to  the  beginning  of  calcification 


DEVELOPMENT    OF    THE    TEETH 


423 


the  enamel  organ  is  in  close  proximity  to  the  dentin  papilla,  the  original 
form  of  the  latter  being  represented  by  the  calcified  dentin. 

We  have  now  arrived  at  that  period  of  fetal  existence  when  it  is  possi- 
ble to  study  the  macroscopic  development  of  the  permanent  first  molar. 
Figure  329  represents  a  section  of  the  lower  jaw  of  a  six  months'  fetus, 

Enamel  Organ  Oral  Mucous  Membrane  or  Gum 


Follicular  Wall 


Tooth-pulp       Floor  of  Tooth-follicle 
Fig.  328. — Dissection  Showing  Pulp,    Calcined  Cap,  and  Enamel  Organ. 


Oral  Mucous  Membrane 


and  displays  not  only  the  sacs  of  the  deciduous  teeth,  but  also  that  of 
the  permanent  first  molar.  In  most  respects  the  evolution  of  this  tooth 
is  similar  to  that  of  the  temporary  organs,  having  its  origin  from  the 
deep  epithelial  layer,  either  directly  or  by  continuation  of  the  tooth- 
band  backward.  Preparations  for  its  growth  are  begun  as  early  as 
the  third  fetal  month,  at  which  time  the  enamel  organ  is  given  off,  and 
thereafter  the  developmental  process  is 
identical  with  that  of  the  deciduous  teeth. 
There  is  one  structure,  however, 
intimately  connected  with  the  develop- 
ment of  the  permanent  teeth  not  found 
in  connection  with  the  deciduous  organs 
— the  gubernaculum,  or  leading  cord. 
Figure  330  represents  another  section  of 
a  six  months'  fetal  mandible,  with  the 
dentin  papilla  for  the  permanent  first  lg'  329' 

molar  turned  out  from  the  follicle  after  being  rolled  from  its  bony  in- 
casement.  Attached  to  the  apex  of  the  tooth-sac  (which  has  been 
turned  back),  and  leading  from  it  to  the  epithelium  of  the  jaw,  is  the 
gubernaculum.  This  fibrous  structure  was  at  one  time  thought  to  be 
directly  concerned  in  the  development  of  the  tooth.  Although  this  is 
denied  at  present,  little  is  said  in  regard  to  its  function,  but  it  undoubtedly 


424 


HISTOLOGY 


serves  the  purpose  of  directing  the  tooth  to  that  position  which  it  should 
occupy  in  the  jaw,  and  where  the  least  resistance  to  its  eruption  is 
formed  by  the  foramen  which  the  cord  has  established.  Each  of  the 
permanent  teeth  is  provided  with  a  similar  membranous  cord,  an  illus- 
tration and  more  complete  description  of  which  will  follow  later  on. 
We  have  now  arrived  at  a  period  when  the  subject  under  consideration 

Oral  Mucous  Membrane     Gubernaculum 


Dentin  Papilla 
Fig.  330. 

naturally  becomes  of  deeper  interest.  I  refer  to  that  time  when  the  being 
changes  from  a  complex  dependent  condition  to  one  of  self-providing  inde- 
pendence. Previous  to  the  time  of  birth  the  teeth  appear  to  be  but  little 
disturbed  by  certain  morbid  conditions  which  might  be  present  in  the  par- 
ent and  from  their  earliest  inception  up  to  this  period  their  development 
proceeds  with  but  little  interruption  and  with  much  regularity.     Figure  33 1 


Hi 

it* 


Fig.  331. — Deciduous  Teeth  at  Birth.     (Reflected  picture.) 

illustrates  the  condition  of  the  deciduous  teeth  at  birth;  the  central  incisors 
are  calcified  externally  to  the  cervical  line,  the  lateral  incisors  to  a  point  cor- 
responding to  the  summit  of  the  palatocervical  ridge;  the  cuspids  have  ad- 
vanced somewhat  beyond  the  angles  of  the  crown,  while  the  molars  have 
their  crowns  calcified  to  about  one-half  their  completed  length.  With  all 
this  progress  as  represented  by  the  external  contour  of  the  tooth-crowns, 


DEVELOPMENT    OF    THE    TEETH 


425 


the  internal  form  appears  to  be  somewhat  slow  in  assuming  the  shape  of 
the  future  pulp-chamber.  From  the  beginning  of  the  saccular  stage  of 
development  up  to  the  time  of  birth  there  is  but  little  increase  in  the 
diameter  of  the  tooth-sac,  but  there  occurs  a  gradual  increase  in  its  length. 
Figure  332  shows  the  mandible  from  a  child  one  week  old,  with  the 
greater  part  of  the  external  or  facial  surface  of  the  bone  removed,  exposing 

Oral  Mucous  Membrane 


Fig.  332. 

not  only  the  sacs  containing  the  developing  deciduous  teeth,  but  also 
that  of  the  permanent  first  molar.  The  relation  of  the  sacs  to  the  inferior 
dental  canal  is  apparent,  as  well  as  the  firm  attachment  of  the  follicular 
walls  to  the  oral  membrane.  In  figure  333  the  tooth-sacs  have  been  dis- 
sected and  the  pulps  removed  from  the  calcified  caps,  presenting  an 
additional  illustration  of  the  amount  of  dentin  deposit  at  this  age.     To 

Calcified  Caps 


Dentin  Papilla  of 
Permanent  First 
Molar 


Pulps 


Fig.  333. 


further  illustrate  the  size  and  form  of  the  pulp  as  compared  with  the 
calcified  cap  at  birth,  a  transverse  incision  was  made  through  the  left  supe- 
rior maxilla,  at  a  point  corresponding  to  the  base  of  the  pulp,  as  shown 
in  figure  334.  The  calcified  parts  remain  in  position  resting  against  the 
remaining  portion  of  the  enamel  organ,  while  the  pulps  are  dislodged  and 
may  be  observed  resting  upon  the  incised  surface.     In  this  illustration 


426 


HISTOLOGY 


the  dentin  papilla  for  the  first  permanent  molar  is  also  seen,  being  sup- 
ported by  the  walls  of  the  follicle,  which  in  turn  are  attached  to  the  oral 


Calcified?Caps,  Resting  against 
Enamel  Organ 


Tooth-pulps'.Rolled  Out 


Section  of  Superior  Maxilla 


Pulp  of  Permanent  First  Molar 

Fig.  334. 


epithelium  by  the  gubernaculum.  It  may  also  be  noted  that  those  parts 
of  the  pulp  corresponding  to  the  cusps  and  marginal  ridges  show  a  decided 
convergence  of  the  surface  toward  the  center. 


J£ 

)entin  Papilla  for 
Permanent  First 
Molar 

Papilla 
Calcined  Cap 

St 

—^r 

Walls  of  Tooth- 
sacs 

3* 

Maxilla 


Oral  Mucous 
Membrane 


Fig.  335. 

Reference  has  been  made  to  the  formation  of  the  follicular  walls  by  a 
differentiation  of  cells,  which  at  an  early  period  are  given  off  from  the 


DEVELOPMENT    OF    THE    PERMANENT    TEETH 


427 


base  of  the  papillae  and  to  the  continuity  of  the  two  structures.     Figure 
335  was  prepared  for  the  purpose  of  showing  these  intimate  relations. 
The  sides  of  the  sacs  were  opened  and  turned  back,  the  calcified  tooth- 
caps  with  pulps  in  position  were  grasped  and  given  several  revolutions, 
thus  twisting  the  remaining  portion  of  the  walls,  the  floor  of  which  is 
seen  as  a  continuous  structure  given  off  from  the  pulp  and  connecting  it 
with  the  epithelium  of  the  jaws. 
• 
PREPARATIONS  FOR  THE  DEVELOPMENT  OF  THE 
PERMANENT  TEETH. 

A  little  before  the  time  of  birth  sufficient  advance  has  been  made  in 
the  development  of  the  permanent  teeth  to  permit  a  study  of  their  rela- 
tions with  the  temporary  organs.  The  early  preparations  for  the  growth 
of  these  teeth  was  for  a  long  time  a  subject  of  much  controversy,  some 
writers  advancing  the  theory  that  the  buds  for  the  permanent  teeth  were 
produced  or  given  off  from  the  sacs  of  the  temporary  teeth;  others  contend- 


Tooth-sac  of  Advancing 
Deciduous  Tooth 


Gum 


Gubemaculum       Tooth-sac  of  Receding  Permanent  Tooth 
Fig.  336. — Section  through  Superior  Maxillae,  Sixth  Month  after  Birth. 

ing  that  the  cords  were  derived  from  the  remnants  of  the  primitive  cords 
immediately  after  their  rupture.  The  theory  now  generally  acceptee 
is  that  the  cord  is  given  off  from  the  primitive  cord  at  a  point  in  clods 
proximity  to  its  attachment  to  the  deciduous  enamel  organ.  This  can 
only  apply  to  those  teeth  which  are  succedaneous,  and,  therefore,  does 
not  include  the  permanent  molars,  as  heretofore  stated.  Whatever 
theory  be  accepted  in  regard  to  the  genesis  of  the  permanent  teeth,  there 
can  be  no  mistake  in  regard  to  that  part  of  the  process  which  we  are 
permitted  to  ocularly  investigate.     I  shall,  therefore,  proceed  to  describe 


428 


HISTOLOGY 


the  position  and  contents  of  the  permanent  tooth-sacs  at  birth.  Figure 
336  presents  the  result  of  a  vertical  dissection  through  the  superior 
maxillary  bones,  the  inner  surface  of  the  right  maxilla  being  exposed 
to  view.  Many  of  the  frail  processes,  particularly  those  entering  into 
the  construction  of  the  nasal  cavities,  were  lost  during  the  preparation 
of  the  specimen,  the  whole  purpose  of  the  dissection  being  to' show  the 
sac  of  the  permanent  incisor  and  its  relationship  to  its  predecessor. 
By  this  time  calcification  in  the  deciduous  incisor  has  so  far  advanced 
that  the  contour  of  the  tooth-crown  may  be  plainly  outlined  through 
the  walls  of  the  sac.     Resting  against  the  lingual  concavity  of  the  crown 


Foramina  for  Gubernacula 


Fig.  337. 

of  the  temporary  incisor  is  the  sac  containing  the  formative  organs 
for  its  permanent  successor.  This  permanent  tooth-sac  does  not  long 
remain  in  such  close  proximity  to  the  deciduous  tooth-crown,  for  as 
the  latter  advances  toward  the  surface  of  the  gum  the  former  recedes 
and  is  soon  inclosed  in  a  separate  crypt,  which,  were  it  not  for  the  guber- 
nacular  foramen,  would  completely  inclose  it.  Figure  337  represents 
a  section  of  the  left  superior  maxilla,  introduced  at  this  point  for  the 
purpose  of  showing  the  position  of  the  foramina  for  the  gubernacula. 
These  may  be  observed  immediately  posterior  to  the  incisor  and  cuspid 
teeth.  At  birth  these  foramina  do  not  exist  as  such,  the  partially  formed 
ed  vaults  containing  the  sacs  for  the  permanent  teeth  appearing  as  an 
extension  of  the  temporary  crypts  in  a  palatal  direction;  but  as  the 


DEVELOPMENT    OF    THE    PERMANENT   TEETH 


429 


temporary  teeth  advance  and  the  permanent  teeth  recede,  the  roof  of  the 
crypt  is  completed,  and  the  foramen  established  by  the  presence  of  the 
gubernaculum.  This  extension  of  the  temporary  crypts  is  more  clearly 
demonstrated  in  figure  338,  which  represents  one  side  of  the  lower  jaw  at 
birth  with  the  partially  calcified  deciduous  teeth  in  position  in  the  bone. 


Tooth-sacs  for  Permanent  Incisors 


Gum 


Tooth-sac  for  Perma- 
nent First  Molar 


Deciduous  Teeth  in  Bony  Crypts 

Fig.  338. 

Suspended  above  this  is  the  gum,  which  has  been  dissected  from  the 
bone,  having  attached  to  its  under  surface  the  sacs  for  the  permanent 
teeth.  Those  for  the  incisors  are  particularly  well  defined  and  their 
place  of  lodgment  in  the  bone  readily  noted.  In  the  case  of  the  cuspid, 
both  the  deciduous  tooth  and  the  permanent  tooth-sacs  are  in  position 
in  the  crypt.     The  cords  which  support  the  incisors  are  somewhat  length- 

Oral  Mucous  Membrane 


Follicular  Wall,  Turned 
Wrong  Side  Out 


Papilla  for  Permanent  First 
Molar 


Dentin  Papillae  for  Permanent  Teeth,  Upside  Down 

Fig.  339. 


ened  from  the  weight  of  the  sacs,  the  gubernacula  not  assuming  this 
thread-like  form  until  the  permanent  sacs  have  further  receded. 

Figure  339  shows  the  result  of  a  dissection  upon  these  permanent 
tooth  follicles.  The  dentin  papillae  of  the  various  teeth  are  seen  in  a 
reversed  position,  with  the  follicular  walls  attached  to  their  bases.  At 
this  period  the  papillae  for  the  permanent  incisors  may  be  compared  to  the 


43° 


HISTOLOGY 


tail  of  a  fish,  being  perfectly  transparent  over  its  free  extremity,  which 
feature  is  gradually  lost  as  its  thickened  base  is  approached.  The  fish- 
tail appearance  is  further  represented  by  the  division  of  the  free  extremity 
into  three  distnct  parts,  each  of  which  provides  a  separate  point  of  calcifi- 


Tooth-sacs  of  Permanent  Teeth 


Tooth-sacs  of 
Deciduous  Teeth 


Lingual  Surface  of  Mandible 

Fig.  340. 

Tooth-sacs  of  Deciduous  Teeth 


Tooth-sac  of  Permanent  Tooth 


Periosteum  and  Mucous  Mem- 
brane from  Hard  Palate 


Gubernaculum 


Tooth-sac  for  Permanent 
First  Molar 


Fig.  341  -Same  as  Figure  340,  Except  on  Upper  Jaw. 

cation.  This  cuspid  papilla  is  missing,  and  the  bicuspids  have  advanced 
little  beyond  the  form  of  the  primitive  bulb.  The  first  molar  is  shown 
with  the  full  diameter  of  the  crown  represented  by  the  pulpal  mass,  and 
the  tips  of  the  cusps  are  already  beginning  to  take  on  the  calcific 
action. 


DEVELOPMENT    OF    THE    PERMANENT   TEETH 


43 1 


A  further  illustration  of  the  progress  of  the  development  of  the  per- 
manent teeth  and  their  relation  to  the  deciduous  organs  may  be  seen 
in  figure  340,  the  dissection  in  this  instance  being  upon  the  lower  jaw  of 
a  one-month-old  child.  The  membrane  has  been  lifted  from  the  bone 
with  the  tooth-sacs  attached  to  it.  Immediately  posterior  to  the  sacs 
containing  the  crowns  of  the  temporary  incisors  and  cuspids  are  those 
for  their  corresponding  successors,  the  papillae  of  which  have  already 
assumed  the  tubercular  outline  of  the  future  cutting-edge.  While  the 
permanent  tooth-sacs  are  distinctively  independent  pouches,  there  ap- 
pears, nevertheless,  to  be  a  well-established  fibrous  connection  existing 


Tooth-sacs  of 
Permanent  Teeth 


Tooth-sacs  of 
Deciduous  Teeth 


Periosteum  of  Hard  Palate 
Fig.  342. — Tooth-follicles  for  Deciduous  and     Permanent  Teeth,  Three  Months 

after  Birth. 


between  the  outer  layer  of  the  two  follicular  walls.  This  fibrous  union 
is  gradually  broken  as  the  permanent  sacs  recede  and  become  incased 
in  their  own  vaults. 

While  the  permanent  tooth-sacs  are  generally  referred  to  as  "reced- 
ing," it  is  a  question  if  this  term  is  fully  justified.  While  the  follicles 
do  not  remain  in  close  relation  with  their  predecessors,  the  change  in  the 
relative  position  of  the  two  is  principally  brought  about  by  the  advance 
in  the  deciduous  sacs,  this  forward  movement  being  accompanied  by  a 
marked  growth  of  the  bone  in  the  direction  of  the  future  alveolar  ridge, 
thus  leaving  the  permanent  tooth-sacs  well  buried  in  the  substance  of 
the  jaw. 

Figure  342  represents  the  result  of  a  dissection  upon  the  superior 
maxillae  of  a  three-months-old  child.  The  mucous  membrane  coveting 
the  hard  palate,  together  with  the  periosteum,  has  been  dissected  from 
the  bones  and  turned  over  for  examination.     The  tooth-follicles  for  all 


432  .  HISTOLOGY 

the  deciduous  teeth,  as  well  as  those  of  the  succedaneous  permanent 
organs,  may  be  observed  firmly  attached  to  the  fibrous  tissue.  The 
permanent  incisor  sacs  at  this  age  are  almost  equal  in  size  to  those  of 
their  predecessors,  and  the  dentin  papillae  within  possess  a  mesio-distal 
diameter  almost  equal  to  those  of  the  calcified  temporary  caps.  The 
sacs  containing  the  germs  for  the  permanent  cuspids  and  bicuspids  are 
somewhat  diminutive,  but  the  enamel  organs  within  are  already  molding 
the  contour  of  the  future  tooth-crowns  upon  the  dentin  papillae. 

By  the  beginning  of  the  second  month  after  birth  calcification  in  the 
crowns  of  all  the  deciduous  teeth  is  about  complete,  and  preparation  for 
growth  of  the  roots  is  under  way.  While  at  this  period  the  tooth-crowns 
may  be  said  to  be  almost  completely  calcified,  this  does  not  apply  to  the 


FiG.  343. 

interior  of  the  crowns,  the  deposit  of  dentin  internally  being  a  continuous 
process,  resulting  in  a  gradual  reduction  in  the  capacity  of  the  pulp- 
cavity.  It  is  also  quite  probable  that  the  enamel  organ  is  somewhat  active 
up  to  the  eruptive  period,  and,  if  this  be  true,  the  enamel  covering  of  the 
crown  is  not  complete  until  this  time.  Whatever  be  the  condition  in  the 
crowns,  the  time  for  the  formation  of  the  roots  has  arrived,  and  it  is  princi- 
pally through  the  activity  of  the  tooth-pulp  that  they  are  generated. 
We  have  seen  that  the  contour  of  the  tooth-crown  was  first  molded  upon 
the  dentin  papilla;  so  it  is  with  the  tooth-root:  by  a  gradual  elongation  of 
the  sac,  accommodations  are  afforded  the  tooth-pulp  for  a  corresponding 
growth.  As  the  pulp  lengthens  out  toward  the  future  apex  of  the  root, 
it  is  molded  to  the  root-form,  and  calcification  takes  place  by  the  genera- 
tion of  odontoblastic  cells  upon  the  periphery  of  this  organic  root-form. 
While  the  process  of  root-formation  in  the  single-rooted  tooth  may  be 
readily  comprehended,  the  bifurcation  or  trifurcation  of  the  molar  roots 
presents  a  complication  which  calls  for  special  reference.  Figure  343 
will  assist  in  explaining  this  phenomenon.  In  the  illustration  three 
deciduous  molar  crowns  are  shown,  two  of  which  are  incased  in  their 


DEVELOPMENT    OF    THE    PERMANENT    TEETH  433 

tooth-sacs,  the  third  being  stripped  of  its  membrane.  The  view  is  directly 
upon  the  base  of  the  tooth-sacs,  immediately  beneath  which  is  the  base 
of  the  pulp.  Up  to  this  period  the  odontoblastic  cells  have  been  gener- 
ating about  the  occlusal  surface  and  lateral  walls  of  the  crown  only, 
but  now  an  accumulation  of  these  cells  is  to  be  found  upon  the  base  of  the 
pulp,  lining  up  in  the  position  of  the  future  root-walls.  This  structural 
change  is  faintly  outlined  in  the  illustration.  By  this  inward  extension 
of  the  odontoblastic  cells  from  various  points  about  the  margins  of  the 
pulp,  and  their  union  near  the  center  of  the  mass,  provision  is  made  for 
the  calcification  of  the  various  roots,  which  process  is  considered  separately 
by  an  extension  and  molding  of  the  pulp  into  two  or  more  divisions. 

Calcification  of  the  Cementum. — While  the  dentin  of  the  root  is 
derived  from  the  tooth-pulp,  the  external  covering  of  the  root  (the  cemen- 
tum) is  generated  from  another  source.  In  every  respect  cementum  is 
closely  allied  to  bone,  and  we  find  its  development  provided  for  in  a 
similar  manner.  As  stated  in  another  part  of  th'X  chapter,  the  tooth- 
sac  is  made  up  of  an  outer  and  an  inner  layer,  both  of  which  are  rich  in 
blood-vessels.  These  membranous  walls  continue  to  invest  the  roots 
of  the  teeth  during  their  upbuilding.  The  outer  layer  of  the  sac  remains  as 
a  permanent  structure  placed  between  the  root  and  the  alveolar  walls, 
forming  the  alveolodental  membrane,  while  upon  the  surf  ace  of  the  inner 
layer  osteoblasts  (cementoblasts)  are  developed,  which  are  speedily 
converted  into  bone  or  cementum.  In  this  process  the  tooth-root  may 
be  compared  to  one  of  the  long  bones  of  the  body,  and  the  development 
of  the  cementum  considered  under  the  head  of  Subperiosteal  Ossification. 
The  only  variation  to  be  observed  between  this  and  subperiosteal  develop- 
ment of  bone  is  in  the  presence  of  a  single  Harversian  canal  (as  the  pulp- 
cavity  may  be  considered),  and  even  this  difference  is  sometimes  over- 
thrown by  small  canals  running  at  right  angles  to  the  pulp.  These  small 
canals  are  generally  found  near  the  apex  of  the  root,  at  which  point  the 
cementum  is  the  thickest.  Like  the  enamel  cap  of  the  tooth-crown,  the 
cementum  is  deposited  upon  the  surface  of  the  dentin  of  the  root,  thus 
increasing  its  diameter.  The  calcification  of  cementum  is  more  fully 
considered  later  on. 

Eruption  of  the  Teeth. — Up  to  this  time  no  reference  has  been 
made  to  that  process  by  which  the  teeth  burst  forth  from  their  bony  incase- 
ments,  and,  penetrating  the  mucous  membrane,  made  their  appearance 
in  the  mouth.  Attention  has  been  called  to  the  growth  of  the  bone 
about  the  tooth-follicles — first  forming  beneath  them  as  an  open  gutter, 
next  surrounding  their  lateral  walls  and  inclosing  each  follicle  in  a  sepa- 
28 


434  HISTOLOGY 

rate  compartment,  and  finally  each  tooth  becoming  more  completely  envel- 
oped by  an  arching-over  of  the  mouth  of  the  bony  vault.  This  condition 
in  the  maxillary  bones  is  reached  between  the  seventh  and  eighth  month 
after  birth,  and,  almost  simultaneously  with  the  completed  incasement 
of  the  teeth  by  the  bone,  active  absorption  begins,  that  portion  of  the 
bone  which  was  last  in  forming  being  gradually  removed.  The  cause 
of  the  absorption  of  the  bone  may  readily  be  attributed  to  the  advance- 
ment of  the  tooth,  but  the  forces  which  are  responsible  for  this  latter 
phenomenon  do  not  appear  to  be  clearly  understood.  In  a  general  way, 
the  advancement  of  the  crown  may  be  said  to  result  from  the  elonga- 
tion of  the  root  by  the  addition  of  dentin  to  its  free  extremity.  But, 
when  it  is  taken  into  consideration  that  the  cuspid  teeth,  both  decid- 
uous and  permanent,  have  their  roots  fully  or  nearly  calcified  before 
they  begin  to  advance  toward  the  surface,  an  exception  to  the  generally 
accepted  theory  is  established.  The  eruptive  process  takes  place  first 
in  the  anterior  teeth,*  and  the  bone  overlying  the  labial  surface  is  first 
removed. 

This  loss  of  the  bony  structure  is  continued  until  fully  one-half 
of  the  labial  surface  is  uncovered,  and,  as  the  crowns  continue  to  advance 
toward  the  surface,  they  assume  a  more  prominent  position  in  the  arch, 
and  thus  their  cutting-edges  become  bared.  The  palatal  or  lingual  face 
of  the  crypt  serves  a  double  purpose,  forming  not  only  a  covering  to  the 
deciduous  tooth,  but  also  serving  the  permanent  tooth-sac  in  the  same 
capacity.  This  part  of  the  crypt  remains  unabsorbed,  the  tooth-crown 
glides  by  its  margin,  and,  after  penetrating  the  mucous  membrane,  makes 
its  appearance  in  the  mouth.  Closely  following  the  absorptive  process 
comes  a  rebuilding  of  the  parts,  until,  finally,  when  the  tooth  is  fully 
erupted,  it  is  firmly  supported  by  the  new  bone  filling  in  about  the  base  of 
the  root.  Accompanying  the  eruption  of  the  anterior  teeth  and  their 
establishment  in  the  arch  is  an  increase  in  the  depth  of  this  portion  of  the 
jaw,  and,  as  the  molars  advance  and  assume  their  position,  there  is 
a  corresponding  increase  in  the  depth  of  the  jaw  in  this  locality.  At  the 
beginning  of  the  eruptive  period  the  roots  of  the  deciduous  teeth  are  but 
partially  calcified,  but  as  the  crowns  advance  the  calcific  action  at  the 
extremity  of  the  roots  is  continued,  and,  in  the  majority  of  instances,  by 
the  time  the  crowns  are  fully  erupted  the  roots  are  completely  formed. 
During  the  period  of  eruption  the  transitory  nature  of  the  alveolar  por- 
tion of  the  jaw-bone  is  made  manifest,  accommodating  itself  to  the 
growth  of  the  teeth  as  well  as  to  their  change  of  position.     The  free 

*See  Description  of  the  Teeth  in  Detail. 


DEVELOPMENT    OF    THE    PERMANENT    TEETH 


435 


margins  of  the  alveolar  walls  are  taking  on  new  structure,  which  ad- 
vances with,  and  becomes  adapted  to,  the  base  of  the  tooth-root.  Coin- 
cident with  this  the  deeper  portion  of  the  alveolar  process  is  formed 
by  a  rapid  filling-in  about  the  root  as  the  tooth  travels  onward  to  assume 
its  final  position  in  the  jaw.  The  eruption  of  the  teeth  is  usually  by 
pairs,  with  a  slight  intermission  between  each  class.  The  central  in- 
cisors first  make  their  appearance,  followed  by  the  laterals,  after  which 
the  first  molars  are  erupted.  The  cuspids  usually  follow  the  first  mo'ars, 
and,  finally,  the  second  molars  take  their  place  in  the  arch.  While 
this  brief  description  of  the  eruption  of  the  teeth  refers  to  the  deciduous 
organs  only,  the  process  in  the  permanent  teeth  is  almost  identical 
with  this.  Further  reference  to  the  eruption  of  the  permanent  teeth 
will  be  made  in  connection  with  the  degeneracy  of  the  temporary  set. 


Fig.  344. — Hard  Palate  from  a  Nine-months-old  Child,  Actual  Size. 


To  return  to  the  subject  of  tooth-development,  attention  is  called  to 
figure  344,  prepared  from  a  dissection  upon  a  nine-months-old  child, 
the  illustration  representing  the  hard  palate,  or  roof  of  the  mouth,  at  this 
period.  The  four  incisor  teeth  have  made  their  appearance,  the  labial 
surfaces  of  the  crowns  being  fully  exposed,  while  those  facing  the  palate 
are  but  slightly  uncovered.  The  approach  of  the  remaining  deciduous 
teeth  is  plainly  indicated  by  the  fullness  of  the  alveolar  borders,  and  the 
margins  of  the  crypts  are  now  being  removed  by  absorption. 

If  the  mucous  membrane  should  be  removed,  the  crowns  of  the 
advancing  teeth  would  be  brought  to  view  after  the  removal  of  the  walls 
of  the  tooth-sacs,  while  the  approaching  cuspids  and  second  molars  yet 
remain  partly  covered  by  an  arching  over  of  the  walls  of  the  crypts; 


436 


HISTOLOGY 


the  resorptive  process  has  also  begun  in  these  parts.  It  would  also  be 
observed  that,  while  the  walls  of  the  crypts  are  molded  to  the  outlines 
of  the  tooth-crowns,  there  exists  a  well-defined  interspace  between  the 
two.  During  the  growth  of  the  tooth  this  interspace  is  filled  by  the  walls 
of  the  tooth-sacs,  and  even  after  the  teeth  have  passed  the  saccular  stage 
of  development,  and  assumed  their  positions  in  the  mouth,  there  yet 
remains  between  the  roots  and  the  alveolar  walls  a  slight  space  which 
is  occupied  by  the  alveolodental  membrane. 

The  next  dissection  is  one  upon  the  superior  maxillae  of  a  two-year- 
old  child  (Fig.  345),  representing  the  roof  of  the  mouth  of  this  subject. 


Fig.  345. — Hard  Palate  from  a  Two-year-old  Child,  Actual  Size. 

In  this  specimen  it  will  be  noticed  that  all  of  the  deciduous  teeth  are 
erupted  with  the  exception  of  the  second  molars.  The  lingual  surfaces 
of  the  incisors  are  fully  uncovered,  while  in  the  cuspids  the  labial  surfaces 
are  much  more  exposed  than  the  lingual.  In  figure  346  the  mucous  mem- 
brane and  sufficient  of  the  bone  have  been  removed  to  expose  the  tooth- 
sacs  of  the  developing  permanent  teeth.  The  dissection  furnishes  no 
additional  information  over  that  obtained  from  figure  345,  excepting 
that  the  primitive  follicles  for  the  permanent  second  molars  make  their 
appearance  at  this  time. 

At  this  early  period  the  jaw  has  not  lengthened  sufficiently  to  permit 
of  these  follicles  occupying  their  future  position;  consequently  they  are 
found  generating  immediately  over  the  tooth-sacs  of  the  first  permanent 
molars.  As  the  first  molars  advance  and  the  jaw  lengthens  backward, 
these  follicles  will  be  carried  to  the  distal  by  the  extension  of  the  mucous 


DEVELOPMENT    OF    THE    PERMANENT    TEETH 


437 


membrane,  to  which  they  are  firmly  adherent.  If  these  follicles  were  to 
be  dissected  at  this  time,  the  papillae  would  be  without  definite  form, 
showing  that  the  early  function  of  the  enamel  organ  has  not  yet  begun. 
The  tooth-sacs  containing  the  permanent  lateral  incisors  are  found 
immediately  beneath  the  palatal  plates,  and  frequently  during  their 
earlier  life  they  are  not  even  protected  by  the  bone,  being  in  immediate 
contact  with  the  mucous  membrane.  On  account  of  the  imperfect 
protection  frequently  afforded  these  sacs,  the  germs  are  sometimes 
injured  and  the  teeth  fail  to  make  their  appearance. 


Sac  for  Perma- 
,  Nnent  Second 
1    -Molar 


Fig.  346.— Roof  of  the  Mouth  of  a  Two-year-old  Child. 

In  figure  347  the  walls  of  the  sacs  shown  in  figure  346  have  been  opened, 
and  the  relations  existing  between  the  first  and  second  dentition  at  the  end 
of  the  second  year  become  apparent.  The  crowns  of  the  permanent  in- 
cisors are  deeply  set  in  the  substance  of  the  jaw,  while  the  partially  calci- 
fied crowns  of  the  permanent  laterals  are  in  close  proximity  to  the  palatal 
surface.  The  partially  formed  crowns  of  the  permanent  cuspids  are  still 
more  deeply  seated  >n  the  substance  of  the  jaw  than  those  of  the  central 
incisors,  and  are  not  visible  in  the  illustration.  In  this  connection  it  will 
be  well  to  again  refer  to  the  gubernaculum,  and  to  its  function — that  of 
directing  the  tooth  to  its  proper  position  in  the  arch.  By  reference  to 
the  illustration  the  crowns  of  the  permanent  teeth  will  be  observed  head- 
ing in  various  directions,  and,  while  in  this  instance  there  appears  to  be  a 
general  tendency  for  them  to  advance  and  assume  their  proper  positions 
in  the  arch,  in  many  cases  they  will  be  found  directed  at  right  angles  to 
the  point  at  which  they  should  emerge  from  the  bone.     The  gubernacu- 


438 


HISTOLOGY 


lum,  which  appears  to  be  nothing  other  than  an  elongation  of  the  follicular 
walls,  not  only  directs  the  tooth  by  the  tension  of  its  fibers,  but  the  fora- 
men which   its   presence  creates  stimulates   absorption   over  the  tract 


Fig.  347. — Development   of  the  Teeth  about  the  Second  Year. 

to  be  traveled  by  the  tooth.  Figure  348  was  prepared  for  the  purpose 
of  better  showing  the  gubernacula,  and  the  manner  of  connecting  the 
tooth-sacs,  with   the   oral   mucous   membrane.     This   condition  is   well 


Guber- 
naculum 


Tooth-sac  for  First  Bicuspid 
Fig.  348-Dissection  of  Lingual  Face  of  Lower  Jaw,  Child  Nine  Months  Old. 


shown  in  the  anterior  teeth,  the  tooth-crowns  having  receded  well  toward 
the  body  of  the  jaw.  The  follicle  for  the  first  bicuspid  may  be  observed 
attached  to  the  lingual  face  of  the  deciduous  molar  sac,  and  the  lead- 


DEVELOPMENT    OF    THE    PERMANENT    TEETH 


439 


ing  cord  is  not  yet  an  adjunct  to  the  developmental  process,  but  this 
structure  will  make  its  appearance  as  the  follicle  recedes  from  the  surface. 
In  figure  349  the  deciduous  molars  have  been  removed  from  the  jaw 
and  the  relations  existing  between 
these    teeth    and    the    developing 
bicuspids    is    shown.     The    tooth- 
follicles  for  the  succedaneous  teeth 
are  found  immediately  beneath  the 
gingival    margin,    and    apparently 
attached  to  the  deep  layer  of  the 
mucous  membrane.     This  relation- 
ship between  the  permanent   and      FlG       349._Deciduous      Molars      with 

temporary  Organs  is  present  about     Tooth-sacs    for    Bicuspids    Attached    to   the 
.  .  •     1       i  1         Gingival  Tissue. 

the    eruptive    period,    but    as    the 

deciduous  tooth  advances  and  the  permanent  tooth-sac  recedes,  the 
two  organs  become  more  widely  separated,  and  the  permanent  follicle 
is  connected  to  the  surface  only  by  the  elongated  follicular  fibers  which 
form  the  gubernaculum. 

Decalcification  of  the  Deciduous  Teeth.— By  the  close  of  the 
second  year  the  twenty  deciduous  teeth  have  taken  their  place  in  the 


Deciduous  Molars 


Deciduous  Incisors 


Permanent 
Incisors 


Permanent 
Cuspid 


Fig.  350. — Same  as  Figure  235,  with  Tooth-sacs  Opened  Showing  Developing 
Teeth  in  the  Jaw. 


dental  arch,  their  roots  have  become  fully  calcified,  and  the  apical  fora- 
mina established;  it  is  only  for  a  short  period,  however,  that  they  remain 
thus  perfect,  the  process  of  decalcification  beginning  about  the  fourth 
year.     This  process  begins  at  the  apical  extremities  of  the  roots  and 


44© 


HISTOLOGY 


gradually  progresses  in  the  direction  of  the  crowns.  Commencing 
about  the  fourth  year  with  the  central  incisor,  decalcification  takes  place 
in  the  teeth  in  the  order  of  their  eruption,  the  lateral  incisor  following  the 
central,  the  first  molar  following  the  lateral,  etc.  By  reference  to  figure 
351  an  approximate  idea  of  the  progress  of  decalcification  may  be  obtained, 
and  it  will  be  observed  that  about  three  years  elapse  from  the  beginning 
of  this  rather  obscure  process  to  its  completion,  and  the  final  casting-off 


Fig.  351. — Development  of  the  Teeth  about  the  Sixth  Year. 

or  shedding  of  the  tooth-crowns.  In  reference  to  the  causation  of  this 
dissolution  of  the  deciduous  teeth,  but  little  appears  to  be  known.  It 
has  been  said  to  result  from  the  presence  and  pressure  of  the  advancing 
permanent  teeth,  but  there  is  no  question  but  that  it  occurs  absolutely 
independent  of  these  organs,  decalcification  frequently  taking  place  when 
from  some  obscure  reason,  one  or  more  of  the  permanent  teeth  are 
absent.     During  the  entire  period  of  root  decalcification,  the  pulp  of  the 


DEVELOPMENT    OF    THE    PERMANENT    TEETH 


441 


tooth,  which  is  also  involved  in  the  destruction,  retains  its  vitality,  but 
with  the  loss  of  vitality  in  the  pulp  absorption  of  the  root  ceases;  so  that 
the  gradual  removal  of  the  root-substance  must  be  considered  as  a  purely 
physiologic  action. 

Figure  351  shows  a  dissection  upon  the  jaws  of  a  six-year-old  child, 
by  a  careful  study  of  which,  a  fair  knowledge  of  the  extent  of  absorption 
in  the  deciduous  teeth  at  this  period  may  be  obtained. 

Advance  of  the  Permanent  Teeth. — By  referring  to  figure  351,  the 
relations  existing  between  the  deciduous  and  the   permanent   teeth    at 


Fig.  352. — The  Completed  Dentition. 

about  the  sixth  year  may  be  noted.  While  the  crowns  of  the  deciduous 
teeth  remain  in  position,  a  part  of  the  space  formerly  occupied  by  their 
roots  is  taken  up  by  the  advancing  crowns  of  the  permanent  teeth,  the 
latter  being  calcined  but  little  beyond  their  cervical  lines.  Between  the 
seventh  and  eighth  years  the  crowns  of  the  deciduous  incisors  are  cast 
off,  and  gradually  the  crowns  of  the  permanent  incisors  force  their  way 
through  the  gum,  the  arch  by  this  time  having  sufficiently  increased  in 
size  to  accommodate  the  additional  width  possessed  by  them.  Previous 
to  this  time,  or  about  the  sixth  year,  by  a  backward  extension  of  the  jaws, 
the  permanent  first  molars  have  erupted,  assuming  a  position  in  the  arch 
immediately  posterior  to  the  deciduous  second  molars.     Between  the 


442  HISTOLOGY 

tenth  and  eleventh  years  the  crowns  of  the  deciduous  molars  are  lost,  and 
the  bicuspids  advance  to  take  their  places.  Uusually  by  the  twelfth  year 
there  has  been  sufficient  increase  in  the  length  of  the  jaws  to  permit  of 
an  additional  tooth,  and  the  permanent  second  molar  gradually  takes  its 
position  immediately  posterior  to  the  first.  Between  the  twelfth  and 
thirteenth  year  the  deciduous  cuspids  are  lost  by  decalcification  of 
their  roots,  and  they  are  succeeded  by  the  permanent  cuspids.  We 
therefore  find,  by  the  fifteenth  year,  fourteen  fully  developed  teeth  occupy- 
ing the  dental  arch  of  each  jaw,  the  full  number,  thirty  two,  or  sixteen 
in  each  jaw,  not  being  present  until  the  eruption  of  the  third  molar, 
which,  like  the  other  teeth  of  this  class,  is  compelled  to  await  accommoda- 
tions by  a  further  increase  in  the  length  of  the  maxillary  bones.  This 
tooth  usually  takes  its  place  between  the  eighteenth  and  twenty-first 
years,  and  thus  completes  the  dentition  (Fig.  239). 

FURTHER  CONSIDERATION  OF  TOOTH 
DEVELOPMENT. 

One  phase  of  the  subject  to  which  special  attention  will  be  given  in 
this  chapter  is  that  which  denotes  the  period  at  which  the  various  events 
take  place.  This  is  a  part  of  the  study  which  is  very  difficult  to  deter- 
mine, and  it  would  appear,  for  this  reason,  if  for  no  other,  that  investiga- 
tors of  recent  years  have  been  perfectly  satisfied  to  accept  the  results 
arrived  at  by  their  predecessors  without  any  apparent  effort  to  qualify  the 
deductions.  It  has  for  a  long  time  been  conceded  that  the  primitive 
changes  which  ultimately  result  in  the  formation  of  a  tooth-germ  are 
first  noted  in  a  heaping  up  of  the  epithelial  cells  over  the  district  repre- 
senting the  surface  of  the  future  jaw.  While  in  many  instances  this  is 
true,  there  are  reasons  why  it  cannot  be  considered  an  essential  feature. 
In  the  first  place,  such  a  condition  is  not  always  present,  as  shown  in  fig- 
ure 353,  a  transverse  section  through  the  primitive  jaw  of  a  human  fetus 
about  the  fortieth  day.  The  tooth  bands  at  A  and  B  have  penetrated 
the  submucous  tissue  for  a  considerable  depth,  but  the  surface  epithelium 
does  not  show  a  greater  thickness  at  these  points  than  it  does  over  the 
general  surface  of  the  cavity. 

Figure  354  shows  a  section  made  in  the  same  direction  upon  a  human 
embryo  about  the  sixtieth  day,  and  while  the  tooth  band  (-4)  has  pene- 
trated the  embryonal  connective  tissue  to  a  greater  depth,  there  is  yet  no 
increase  in  the  thickness  of  the  epithelium,  but  rather  a  disposition  for 
the  parts  to  become  depressed.     Another  reason  why  the  heaping  up  of 


TOOTH    DEVELOPMENT 


443 


the  superficial  layer  of  cells  forming  the  embryonal  mucous  membrane 
should  not  be  considered  the  first  sign  of  the  preparation  for  tooth  develop- 
ment lies  in  the  fact  that  these  cells  are  not  directly  interested  in  the 
process,  but  that  the  inflection  of  cells  which  results  in  the  formation  of 
the  tooth  band  results  from  the  deep  or  infant  layer  of  cells  known  as 
Malpighi's  layer,  as  shown  at  B  (Fig.  354). 


Fig.  353. — Transverse  Section  through  Primitive  Jaws  of  Human  Fetus. 

There  is  no  question  but  that  the  location  from  which  the  section  is 
taken  has  much  to  do  with  the  character  and  thickness  of  the  older  layer 
of  epithelial  cells;  and  that  they  do  at  certain  points  constitute  an  epithe- 
lium exceeding  in  thickness  that  of  other  parts  of  the  cavity,  but  this 
condition  most  frequently  occurs  after  the  enamel  organ  has  assumed 
definite  proportions. 


444 


HISTOLOGY 


After  the  formation  of  the  tooth  band,  which,  it  must  be  remembered, 
encircles  the  entire  jaw  in  the  form  of  a  well-defined  body  of  oval  epithelial 
cells  from  the  infant  layer  (shown  at  A  in  cross-section,  Fig.  355),  the 
next  step  in  the  process  is  one  which  concerns  the  location  for  the  individ- 
ual buds  for  the  enamel  organs  of  the  various  teeth,  and  the  approximate 
time  at  which  these  appear. 

In  the  human  subject  we  find  ten  such  spots  appearing  upon  the 
lamina  given  off  from  the  lingual  face  of  the  tooth  band.     These  do  not 


Fig.  354. — Transverse  Section  through  Lower  Jaw,  Human  Embryo,  Sixtieth  Day. 

appear,  however,  at  the  free  extremity  of  the  band,  but  at  some  little 
distance  toward  the  surface  from  this  point,  as  shown  in  figure  356.  In 
this  section  the  tooth-germ  is  severed  from  the  surface  epithelium,  but 
this  is  not  a  true  condition  at  this  period,  as  it  still  retains  its  connection 
with  the  surface  by  a  narrow  band  of  cells,  the  neck  of  the  enamel  organ. 
Two  distinct  classes  of  cells  are  now  (sixtieth  day)  concerned  in  the 
process  of  tooth  development,  those  at  A  being  of  epithelial  origin  and 


TOOTH   DEVELOPMENT 


445 


forming  the  future  enamel  organ,  while  at  B  an  aggregation  of  cells  from 
the  mesoblast  provides  for  the  generation  of  the  pulp  and  dentin.  At  C 
the  narrow  band  of  cells  which  should  continue  to  the  surface  is  shown, 
while  the  free  extremity  of  the  same  body  of  cells  at  D  will  persist  and 
eventually  become  the  germ  for  the  succeeding  tooth.  In  regard  to  the 
time  at  which  the  buds  for  the  various  teeth  appear,  it  might  be  expected 
that  the  same  variation  which  follows  the  development  and  eruption  of 


Fig.  355. — Section  through  Tooth  Band  of  Human  Embryo. 


the  teeth  throughout  would  obtain,  but  such  is  not  the  case,  the  buds  for 
the  deciduous  incisors  appearing  about  the  sixtieth  day,  while  the  germs 
for  their  permanent  successors  are  but  little  later  in  forming. 

By  referring  to  figure  347,  which  shows  a  longitudinal  section  through 
the  lower  jaw  about  the  twelfth  week,  the  deciduous  incisor  is  seen  well 
outlined  by  its  formative  cells,  while  immediately  to  the  lingual  appears  a 
section  of  the  germ  for  the  permanent  cuspid.  Notwithstanding  this, 
there  elapses  a  period  of  several  years  between  the  eruption  of  these  teeth. 


446 


HISTOLOGY 


The  same  relative  progress  will  be  noted  between  the  first  and  second 
teeth,  be  the  subject  human  or  otherwise;  requiring  many  years  to  com- 
plete dentition,  in  the  former,  while  in  most  of  the  lower  animals  the  same 
process  occupies  but  a  comparatively  short  time. 

The  next  stage  in  the  development  of  a  tooth  to  which  attention  will 
be  called  is  that  in  which  the  entire  tooth-crown  is  outlined  by  the  dentin 
papilla  and  surrounded  by  its  epithelial  cap,  the  enamel  organ.     Such 


Fig.  356. — Tooth-germ,  Embryo  Lamb,  Corresponding  to  Sixtieth  Day  (Human).      X40. 


an  advance  in  the  process  is  shown  in  figure  357,  together  with  the  sur- 
rounding structure.  When  this  stage  is  reached,  the  individual  cells 
of  the  tooth-germ  are  strongly  differentiated,  and  the  odontoblasts  are 
making  their  appearance  about  the  summits  of  the  cusps. 

Up  to  this  time  the  cells  present  are  those  which  result  in  the  forma- 
tion of  but  two  of  the  calcified  tooth  tissues,  but  now  there  is  a  marked 
disposition  upon  the  part  of  the  periosteum  of  the  jaw  to  pass  down  by 
the  side  of  the  enamel  organ  (A),  this  being  the  first  indication  of  the 


TOOTH    DEVELOPMENT 


447 


formation  of  the  tooth  follicle,  the  alveolodental  membrane,  and  cemen- 
tum.  At  this  period  it  will  be  observed  that  there  appears  in  this,  the 
molar  region,  a  "heaping  up"  not  only  of  the  surface  epithelium,  but 
also  of  the  connective  tissue  as  well. 

Figure  358  shows  a  section  through  the  growing  mandible  taken 
from  a  portion  of  the  tissue  not  occupied  by  a  tooth-germ.  This  is  of 
interest,  first,  as  giving  a  view  of  the  detached  tooth  band  in  cross-section, 


Fig.  357. — Tooth-germ,  Premolar,   Embryo  Lamb.      X40. 

at  A;  second,  by  showing  the  distribution  of  the  periosteum  to  the  interior 
of  the  jaw  to  serve  the  double  function  of  the  future  tooth-sac  and  alveolo- 
dental membrane;  and,  third,  the  thickened  epithelium  with  the  underly- 
ing tissue  pushing  into  it. 

Although  the  germ  for  the  second  tooth  may  be  observed  at  a  period 
somewhat  prior  to  this,  a  study  of  some  of  its  characteristics  is  best  made 
at  this  time.  The  fact  has  already  been  referred  to  that  this  interesting 
phenomenon  occurs  soon  after  or  even  simultaneously  with  that  for  the 


448 


HISTOLOGY 


first  tooth,  a  portion  of  the  primitive  cord  for  the  latter  persisting  as  the 
germ  for  the  former. 

In  figure  359  the  cells  forming  the  primitive  germ  for  the  enamel 
organ  of  one  of  the  permanent  teeth  are  shown  highly  magnified.  It 
will  be  observed  that  they  are  of  tte  simplest  epithelial  character,  and 


Fig.  358. — Section  through  Jaw  of  Embryo  Lamb,  in  a  District  not  Occupied  by 
a  Tooth-germ.      X40. 


that  they  are  derived  directly  from  the  enamel  organ  of  the  pre-existing 
tooth,  on  the  one  hand,  while,  on  the  other,  they  communicate  with  the 
surface  by  a  narrow  band  of  cells.  In  this  way  it  is  for  a  time  dependent 
upon  both  of  these  parts  for  continuance  and  growth,  but  after  a  time 
it,  too,  like  its  predecessor,  severs  its  connection  with  the  surface,  but 


TOOTH    DEVELOPMENT 


449 


remains  intact  with  the  epithelial  cells  of  the  former  enamel  organ  until 
these  cells  begin  to  atrophy. 

The  cells  which  made  up  this  primitive  germ  are  of  three  varieties: 
the  inner  layer,  or  those  derived  from  the  epithelium  of  the  enamel  organ 
of  the  first  tooth,  being  small  and  spheroidal;  those  of  the  outer  layer, 


Fig.  359. — Primitive  Bud  for  Enamel  Organ  of  Permanent  Tooth,  Human 
Embryo.      X300. 

which  spring  from  the  surface  epithelium,  being  proportionately  larger 
and  cylindrical  or  oblong;  while  those  which  intervene  are  markedly 
irregular  in  outline.  In  this  respect — that  is,  in  the  character  of  the  early 
cell  layers — the  tooth-germs  for  the  permanent  teeth  differ  from  those 
of  the  deciduous. 

The  question  of  the  origin  of  those  teeth  which  have  no  predecessors 
is  one  upon  which  there  has  always  been  more  or  less  discussion,  some 
29 


45° 


HISTOLOGY 


writers  contending  that  they  are  derived  directly  from  the  oral  epithelium 
by  a  special  generation  of  cells  for  each  tooth,  while  others  are  of  the 
opinion  that  as  the  jaw  grows  backward  certain  changes  take  place  which 
result  in  the  establishment  of  an  epithelial  fold  or  lamina,  in  every  partic- 

j&  ML  Mk    jm^ 


Fig.  349. — Diagram  of  Various  Forms  of  Hare-lip  following  the  Lack  of  Union  of  Various 
Processes  Participating  in  the  Formation  of  the  Mouth. 

ular  corresponding  to  the  tooth  band  of  the  deciduous  teeth.  With  these 
two  conflicting  opinions  in  mind,  a  number  of  sections  were  made  through 
the  extreme  distal  end  of  the  jaw.  The  result  favored  the  latter  theory, 
for  here  the  tooth  band  is  seen  similar  in  form  and  location  to  that  ob- 
served in  the  jaw  in  those  locations  from  which  succedaneous  teeth  result. 


CHAPTER  X. 

Anomalies  of  the  Teeth. 

It  has  always  been  conceded  that  the  dental  organs  of  man  are  sus- 
ceptible of  much  variation  in  form  and  structural  arrangement,  and 
that  frequently  this  variation  is  so  positive  that  the  organ  is  pronounced 
anomalous  in  character.  Just  where  the  line  of  distinction  between  the 
normal  and  abnormal  should  be  drawn  is  a  subject  worthy  of  some  con- 
sideration. Some  authorities  define  the  word  anomaly  as  a  marked 
deviation  from  the  normal,  while,  in  the  opinion  of  others,  a  much  broader 
meaning  is  accorded  it;  and  we  find  all  those  conditions-  which  are  in 
themselves  an  irregularity  from  the  typical  structure  or  occurrence   in- 


Fig.  361. — Anomalous  Teeth. 

eluded  in  this  category.  Under  the  first  definition  a  given  structure  or 
organ  is  accorded  a  wide  field  for  its  normal  existence,  while  under  the 
latter  but  slight  deviation  is  necessary  to  classify  it  among  the  abnormal. 

Upon  first  thought  it  would  appear  that  the  ability  or  inability  of  a 
tissue  or  an  organ  to  perform  its  special  function  should,  in  a  measure, 
decide  the  question  of  the  nature  of  its  being,  and  no  doubt  to  a  certain 
extent  this  is  true;  but  while  the  action  of  an  organ  or  a  part  of  the  body 
may,  by  observation,  appear  entirely  satisfactory,  it  is  only  so  at  the 
expense  of  other  organs  or  tissues,  and  these  in  the  course  of  time,  by  this 
extra  exertion,  become  hypertrophied  or  in  other  ways  pathologic. 

While  this  is  especially  applicable  to  those  organs  or  tissues  which 
have  a  wide  range  of  function,  it  may  with  a  good  deal  of  force  be  applied 

45 1 


452  ANOMALIES 

to  the  dental  organs  and  their  immediate  environments.  Anomalous 
conditions  in  the  teeth  may  originate  in,  or  be  confined  to,  one  or  more 
of  the  tooth  tissues,  in  any  of  which  the  structural  disarrangement  may 
eventually  result  in  the  death  or  degeneracy  of  the  part.  Enamel  mal- 
formation is  of  such  a  character  that  it  may  be  observed  upon  the  surface 
either  in  the  form  of  a  multiplication  of  cusps,  or  by  an  extra  development 
of  the  various  ridges  formed  by  pronounced  folds  of  this  tissue.  But 
probably  the  most  disastrous  anomaly  of  the  enamel,  and  one  frequently 
responsible  for  the  downfall  of  this  tissue,  is  found  in  some  defect  of  its 
structural  arrangement  other  than  those  just  referred  to.  In  some 
instances  the  enamel  rods  of  a  given  district,  instead  of  being  normally 
distributed  by  assuming  a  direction  principally  at  right  angles  to  the  long 
axis  of  the  tooth  crown,  are  arranged  without  regard  to  the  base  or  per- 
iphery of  the  tissue,  and  we  have  as  a  result  an  anomaly  of  structure. 
The  question  of  normal  and  abnormal  rod  distribution  now  presents 
itself,  because  in  certain  locations — i.e.,  the  summits  of  the  cusps — 
an  arrangement  of  the  rods  similar  to  that  referred  to  is  so  common  that 
it  may  be  considered  a  normal  condition,  while,  if  a  like  distribution  was 
found  in  other  locations,  the  tissues  should  properly  be  considered 
abnormal. 

Malformed  teeth,  in  respect  to  the  number  and  forms  of  the  cusps 
present,  are  not  alone  confined  to  the  enamel,  but  also  to  the  dentin 
which  first  records  the  tooth  form  on  its  periphery. 

Anomalies  in  the  general  contour  of  the  tooth  crown  are  usually 
confined  to  the  incisors  and  third  molars,  both  the  dentin  and  the  enamel 
contributing  to  the  deformity.  Here  the  defect  is  usually  so  pronounced 
that  but  little  difficulty  is  experienced  in  properly  classifying  the  organ. 
One  of  the  most  frequent  variations  in  form  met  with  in  these  locations 
is  found  in  the  peg-shaped  or  cone-shaped  crown.  If  it  were  possible 
it  would  be  interesting  to  trace  the  development  of  such  a  malformation; 
but  with  our  present  knowledge  of  this  process  in  general,  there  is  little 
doubt  as  to  its  origin,  the  enamel  organ  failing  to  fulfill  its  early  and  pri- 
mary function  of  moulding  the  tooth  crown  in  the  dentin  papilla,  the 
responsibility  for  this  resting  in  the  special  cells  composing  it,  as  well  as 
the  so-called  stellate  reticulum,  which,  it  is  believed,  exerts  a  controlling 
influence  over  the  form  of  the  enamel  cap. 

While  the  organic  defects  of  tooth  crowns  are  numerous  and  varied, 
those  which  are  confined  to  the  roots  are  most  frequent,  in  many  instances 
interfering  to  some  extent  with  the  function  of  the  organ.  When  a  given 
peculiarity  is  confined  to  this  portion  of  the  tooth,  it  is  frequently  difficult 


SUPERNUMERARY    TEETH  453 

to  discriminate  between  the  normal  and  the  abnormal.  Certain  teeth 
are  recognized  as  normal  when  either  a  single  root  or  two  roots  are  pres- 
ent, and  the  acceptance  of  this  fact  increases  the  difficulty  of  a  proper 
classification  of  its  peculiarities. 

In  very  rare  instances  do  we  find  the  roots  of  the  cuspidate  teeth 
more  or  less  crooked;  yet,  at  the  same  time,  many  decidedly  crooked 
roots  are  considered  within  the  natural  law;  while,  on  the  other  hand, 
roots  with  but  little  more  deflection  are  classed  as  anomalous. 

Marked  flexions  of  roots  or  crowns,  cases  of  fusion  or  concrescence, 
are  usually  so  positive  in  character  that  an  anomalous  condition  is  at 
once  acknowledged.  While  tooth  anomalies  are  usually  referred  to  as 
external,  or  as  belonging  to  the  hard  tissues  of  the  organ,  they  are  not 
infrequently  found  in  the  pulp  or  pulp  cavity.  This  cavity,  normally 
following  the  external  contour  of  the  tooth,  is  subject  to  much  variation 
in  outline  and  capacity,  regardless  of  those  changes  which  are  incident  to 
the  continuous  process  of  dentinification. 

Among  these  are  a  complete  division  of  the  pulp  chamber;  horn- 
like processes  penetrating  the  dentin  in  the  direction  of  the  occlusal  sur- 
face in  locations  where  they  would  be  least  expected;  an  unusual  number, 
or  a  peculiar  distribution  of  the  canals,  etc. 

Lack  of  Dentition. — Cases  in  which  there  is  a  total  absence  of 
teeth  have  been  reported.  Guilford  reports  the  case  of  a  man  fifty  years 
of  age  who  never  had  teeth,  the  jaws  not  differing  in  appearance  or  form 
from  those  of  a  person  whose  teeth  have  been  extracted.  The  mother 
of  the  subject  had  the  usual  number  of  teeth,  but  the  grandmother  and 
an  uncle  were  both  edentulous  and  hairless  from  birth.  J.  Tomes 
mentions  two  cases  having  been  reported  to  him,  and  Linderer  mentions 
one. 

On  the  other  hand,  instances  of  a  third  dentition  have  been  reported,, 
but  there  is  great  possibility  of  such  observation  being  erroneous.  Teeth 
belonging  to  the  permanent  set  which  may  have  remained  unerupted 
for  years  could  readily  be  mistaken  as  predecessors  of  a  third  set,  when 
in  after  years  they  made  their  appearance. 

Supernumerary  Teeth. — All  teeth  appearing  in  the  mouth  in  addition 
to  the  normal  number  are  designated  as  supernumerary  teeth.  These  are 
divided  into  two  classes,  those  normal  in  size  and  form  and  those  abnormal 
in  size  and  form.  The  first-named  are  most  likely  to  be  of  the  simple 
class,  incisors  and  cuspids,  and  they  may  occupy  a  regular  position  in 
the  arch  or  may  be  found  inside  the  arch  closely  associated  with  teeth  of 
the   same  type.     Supernumerary   bicuspids   and    molars   are   sometimes. 


454  AMONALIES 

present  cither  in  regular  position  in  the  arch  or  inside  of  it.  When  the 
jaws  are  long  enough  to  accommodate  them,  an  additional  molar  may 
appear  back  of  the  third  molar,  making  four  molars  instead  of  three, 
and  cases  in  which  two  extra  molars  were  thus  placed  have  occasionally 
been  met  with. 

Supernumerary  teeth  of  the  second  class  or  those  which  are  abnormal 
in  size  and  form  are  usually  inclined  to  be  cone-shaped  and  small  in  size, 
this  in  respect  to  the  root  as  well  as  the  crown.  Supernumerary  teeth 
of  this  charater  are  usually  found  in  the  incisor  region,  but  it  occasionally 
happens  that  they  are  found  in  the  molar  district,  but  here,  instead  of 
having  a  single  cone  for  the  crown,  they  are  mostly  made  up  of  a  number 
of  smaller  cones  resembling  many  small  cusps  on  the  occlusal  surface. 
The  number  of  supernumerary  teeth  may  vary  from  one  to  eight  or  ten. 
In  the  latter  instance  they  are  usually  scattered  through  the  entire  alveo- 
lar border  of  the  hard  palate.  As  many  as  ten  or  twelve  teeth  thus 
located  have  been  reported. 

Again,  certain  teeth  are  frequently  missing  from  the  arch.  This 
may  be  occasioned  by  delayed  eruption,  or  it  may  be  the  result  of  im- 
proper activity  within  the  tooth-germ  itself,  so  that  the  tooth  may  have 
failed  to  develop.  The  teeth  most  frequently  missing  are  the  upper 
lateral  incisors.  The  probable  reason  for  this  lies  in  the  fact  that  the 
germs  for  these  teeth  are  located  very  near  the  surface  of  the  bone,  and 
in  some  instances  they  are  not  even  protected  by  a  thin  layer  of  bone 
over  the  follicle.  Being  thus  situated,  they  are  more  or  less  exposed  to 
violence  sufficient  to  destroy  the  germs  and  thus  render  development  of 
the  teeth  impossible.  Lack  of  certain  teeth  in  the  mouth  appears  to  be 
to  some  extent  an  hereditary  feature,  the  ondition  being  transmitted 
from  parent  to  child.  Anomalies  as  to  the  size  of  individual  teeth  are 
frequently  noticed.  When  this  is  the  case,  it  does  not  usually  include 
the  entire  set,  but  is  confined  to  one  or  two,  usually  to  the  same  teeth 
on  each  side.  Teeth  that  are  above  the  usual  size  are  generally  found 
in  persons  of  large  built,  but  if  all  the  teeth  are  proportionate  in  size 
they  cannot  be  included  within  the  abnormal  class.  The  upper  incisors 
are  most  frequently  affected  in  this  way.  Cases  have  been  reported  in 
which  the  central  incisors  in  the  upper  jaw  have  been  fully  twice  the  size 
which  they  should  normally  have  been,  and  all  the  remaining  teeth  in 
the  mouth  perfect  as  to  shape  and  size.  Accompanying  the  abnormal 
condition  it  is  usual  to  find  the  teeth  thus  affected  more  or  less  abnormal 
in  outline,  but,  notwithstanding  this,  retaining  their  form  sufficiently 
well  to  permit  their  proper  classification. 


SUPERNUMERARY    TEETH 


455 


There  are  likewise  teeth  that  are  deficient  in  size.  This  does  not 
refer  to  teeth  all  relatively  small,  as  frequently  found  in  persons  of  small 
frame  and  stature.  This  anomalous  condition  is  often  present  in  the 
upper  lateral  incisors  and  in  the  third  molars.  Teeth  that  are  deficient 
in  size  generally  possess  their  normal  shape. 

While  the  crowns  of  the  teeth  are  susceptible  to  the  above  variations, 
the  roots  appear  to  be  anomalous  much  more  frequently  and  to  a  more 
marked  extent  than  are  the  crowns.  Most  important  among  these  may 
be  mentioned  flexions  of  the  roots.  Curvatures  in  the  roots  may  be 
found  either  in  single-rooted  teeth  or  in  multi-rooted  teeth,  and  the  point 
of  flexion  may  be  located  either  at  the  center  of  the  root  or  near  its  apex, 


Fig.  362. — Anomalous  Roots. 

or  both  of  these  points  may  be  affected.  Flexions  in  the  roots  of  the 
teeth  present  a  great  variety  in  form.  A  single  curve  in  one  direction 
may  be  present  or  a  number  of  curvatures  in  different  directions.  In 
multi-rooted  teeth  the  roots  may  be  so  flexed  that  they  will  entwine  about 
each  other,  and  the  overlapping  portions  may  be  or  may  not  be  fused. 
Probably  the  most  important  cause  for  the  flexions  of  the  roots  of  the 
teeth  is  delayed  eruption,  this  being  particularly  true  if  some  positive 
force  prevents  the  progress  of  the  organ.  The  roots  of  the  teeth  are 
frequently  anomalous  in  regard  to  number.  These  may  have  the  same 
general  form  and  the  same  approximate  length  as  the  normal  root  or 
roots  would  be,  but  they  are  proportionately  smaller  in  diameter.  Cases 
are  on  record  in  which  the  incisor  teeth  have  had  two  distinct  roots,  but 
probably  the  most  frequent  location  for  the  multiplicity  of  roots  is  found 
in  connection  with  the  third  molar.  While  this  tooth  when  normally 
developed  as  to  crown  is  usually  supported  by  three  roots,  it  sometimes 
possesses  five  or  six  smaller  roots. 


456 


ANOMALIES 


It  not  infrequently  happens  that  the  roots  of  the  teeth  are  less  in 
number  than  they  should  normally  be.  This  is  usually  brought  about 
by  the  blending  of  the  roots,  which  occurs  during  the  developmental 
stage.  This  may  be  of  two  distinct  kinds.  In  the  first  place,  it  may 
be  the  result  of  the  conversion  of  two  or  more  pulp  canals  into  one,  or  the 
individual  roots  may  be  united  by  a  body  of  cementum  being  interposed 
between  them.  In  the  former  instance  a  single  pulp  canal  is  mostly 
found  within  the  blended  roots,  while  in  the  latter  the  number  of  canals 
is  usually  normal.  With  this  blending  there  is  generally  a  line  of  demar- 
cation between  individual  roots  as  they  should  normally  exist,  in  the 


Fig.  363. — Fusion  of  Molars 


way  of  longitudinal  depressions  extending  through  the  entire  root  from 
cervical  line  to  apex.  This  anomaly,  like  that  of  the  multiplicity  of  roots, 
is  principally  confined  to  the  third  molars,  and  is  more  frequent  in  the 
upper  jaw  than  in  the  lower. 

Fusion  and  Concrescence. — The  union  of  two  or  more  teeth  is 
known  as  fusion  or  concrescence,  and  may  occur  either  during  the  develop- 
ment of  the  organ  or  after  this  process  has  been  completed.  When  union 
takes  place  during  development,  it  is  characterized  as  fusion;  when  it  takes 
place  after  the  completion  of  this  process,  it  is  known  as  concrescence. 
There  seems  to  be  but  little  doubt  that  fusion  of  the  teeth  occurs  through 
some  irregularity  in  the  tooth-germ  or  germs,  the  beginning  of  the  develop- 
mental process  taking  place  generally  between  two  germs  and  continuing 
together  until  the  complete  calcification  of  the  organs.  Teeth  thus 
united  may  have  an  internal  anatomy  corresponding  in  nearly  every 
respect  to  two  separate  teeth,  and,  on  the  other  hand,  they  may  possess 


FUSION    AND    CONCRESCENCE 


457 


but  a  single  pulp  chamber  and  canal.  Fusian  may  take  place  in  the 
roots  of  the  teeth  alone,  when  it  is  called  partial  fusion;  but  when  it  is 
confined  to  the  roots  and  crowns  alike,  it  is  classified  as  complete  fusion. 
The  teeth  most  likely  to  be  affected  in  this  way  are  the  upper  incisors 
and  the  second  and  third  molars.  Some  distinction  must  be  made  be- 
tween those  teeth  which  are  united  by  a  layer  of  cementum,  and  those  of 


Fig.  364. — Fusion  and  Concrescence. 


Fig.   t,6>. — Fusion  and  Concrescence. 


true  fusion,  the  latter  existing  only  when  there  has  been  a  union  between 
dentin  and  dentin. 

Concrescence. — In  concrescence,  the  roots  of  the  teeth  only  can  be 
affected,  as  the  union  takes  place  after  the  complete  development  of  the 
organ.  The  roots  of  one  tooth  become  united  to  the  roots  of  another 
through  an  additional  growth  of  cementum,  this  growth  being  sufficiently 


45« 


ANOMALIES 


extensive  to  cause  absorption  of  the  alveolar  septa  by  pressure.  Follow- 
ing this  there  is  a  resorption  of  the  pericementum.  The  cemental  tissues 
of  the  teeth  are  then  brought  in  contact,  and  coalescence  gradually  takes 
place.  Concrescence  may  take  place  not  only  between  the  roots  of  two 
teeth,  but  between  the  various  roots  of  an  individual  tooth,  this  resulting 
in  the  same  manner  as  above  described  by  the  destruction  of  the  septa 
within  the  tooth  socket.  When  concrescence  takes  place,  the  process 
is  usually  confined  to  the  apices  of  the  roots,  although  they  may  become 
coalesced  throughout  their  entire  length.  The  teeth  most  commonly 
affected  by  concrescence  are  the  molars  and  bicuspids,  from  the  fact 
that  the  alveolar  walls,  particularly  the  septa  about  these  teeth  are  espe- 
cially thin,  owing  to  the  form  and  relative  location  of  the  roots. 


Fig.  366. — Geminous  Tooth. 

Geminous  Teeth  (Fig.  366). — It  occasionally  happens  that  two 
separate  germs  are  confined  within  a  single  sacculus,  and  from  this 
results  two  teeth,  either  similar  or  dissimilar  in  size  and  form.  One  of 
the  pair  may  be  normal  as  to  form  and  size,  while  the  other  may  be  much 
below  the  normal  size,  but  more  or  less  perfect  in  outline.  Geminous 
or  twin  teeth  may  be  united  or  entirely  separate.  This  condition  is 
most  frequently  found  in  the  molar  teeth,  although  cases  in  which  the 
bicuspids  and  incisors  have  been  thus  affected  are  recorded.  Teeth 
thus  formed  must  not  be  confounded  with  those  in  which  fusion  is  the 
anomaly.  In  geminous  teeth  a  single  sac  contains  two  tcoth-germs 
from  which  results  two  similarly  formed  teeth;  in  fusion  two  follicles 
coalesce,  each  of  which  contains  its  own  germ. 

Besides  the  foregoing,  the  roots  of  the  teeth  are  subject  to  anomalies 
in  size  in  some  instances  being  abnormally  small,  in  others  abnormally 
large.     In  the  former  they  may  nearly  always  be  characterized  as  anomal- 


GEMINOUS    TEETH 


459 


ous;  but  this  is  not  always  true  of  the  latter.  When  the  roots  are  abnor- 
mally large,  it  is  somewhat  difficult  to  discriminate  between  an  anomalous 
and  a  pathologic  condition,  the  latter  usually  being  the  case  when  the 


Fig.  367. 


Fig.  368. 

roots  of  individual  teeth  are  affected.  Roots,  to  be  considered  anomal- 
ous in  size  should  in  a  measure  retain  their  normal  form,  cases  of  hyper- 
trophy (hypercementosis)  usually  resulting  in  the  destruction  of  the 
normal  contours  (Figs.  367  and  368). 


INDEX. 


Accessory  palatal  foramina,  18 
Achromatic  spindle,  241 
Achrornatin  substance,  238 
Acidophiles,  273 
Adipose  tissue,  36 
Adrenal,  282 
Adventitia,  276 
Alveolar  process,  44 

development  of,  48 
Alveoli,  45 
Alveolodental  membrane,  76,  297,  366 

blood  supply  to,  299 

cells  of,  367 

fibers  of,  369 

histology  of,  366 

interfibrous  elements  of,  372 

nerve  supply  to,  299 
Amelification,  324 
Ameloblasts,  323,  325,  394 
Amoeba,  242 

Amoeboid  movement,  268 
Anatomy,  macroscopic,  233 

microscopic,  233 
Angioblasts,  270 
Angular  artery,  13 
Anomalies  of  the  teeth,  451 
Anterior  palatal  foramen,  18 

nerve,  18 

superior  dental  nerve,  100 
Antrum  of  Highmore,  48 
Apical  foramina,  description  of,  190 
Areolar  tissue,  35 
Artery,  deep  facial,  18,  32 

inferior  dental,  96 

internal  maxillary,  18,  107 

lingual,  32 

ranine,  32 

structure,  276 

sublingual,  32 
Axon,  263 
Azygos  uvula;,  23 

Basophiles,  273 
Bicuspid,  lower  first,  172 
second,  175 
upper  first,  125 
second,  135 
Blastoderm,  247 

layers  of,  247 
Blood  and  lymph,  234,  326 
cells,  267 
corpuscles,  267 
development,  267 
physiology,  267 


Blood  plasm,  266 

platelets,  273 
structure,  267 
course  of,  from  heart  to  cheeks,  15 
from  heart  to  hard  palate,  18 
from  heart  to  lips,  8  H 

from  heart  to  soft  palate,  23 
from  heart  to  tongue,  ^t, 
supply  to  the  teeth,  94 
vessels,  274 
Bone,  255 

canaliculi  of,  256 
cells  of,  256 

Haversian  canals  of,  256 
histologic  examination  of,  256 
hyoid,  25,  62 
inferior  maxillary,  54 
lacuna;  of,  256 
marrow  of,  258 
matrix  of,  255 
osteoblasts  of,  258 
palate,  50 
periosteum  of,  258 
Sharpey's  fibers  of,  257 
superior  maxillary,  37 
Bones,  of  the  mouth,  37 

superior  maxillary,  37 
Branchial  arches,  374 
Brown  striae  of  Retzius,  318 
Buccal  cavity,  376 

embryology  of,  375 
glands,  300,  307 
orifice,  2 
Buccinator  muscle,  9 

Calcification,  beginning  of,  419 

of  cementum,  358,  433,  453 

of  dentin,  340,  419 

on  enamel,  324 
Canal,  inferior  dental,  55 

infra-orbital,  38 

posterior  palatal,  41 
Canaliculi,  256 
Canals,  Haversian,  256 

pulp,  190 
Canine  eminence,  38 

fossa,  38 
Capillaries,  274 
Capsular  ligament,  67 
Cartilage,  253 

cells  of,  254 

elastic,  254 

fibro-,  254 

hyaline,  254 


461 


462 


INDEX 


Cartilage,  matrix  of,  253 

Meckel's,  60,  384,  412 

permanent,  255 

temporary,  255 

varieties  of,  253 
Cell,  amitosis,  243 

conductivity,  240 

definition,  236 

division,  242 

fat,  253 

gland,  253 

growth,  239 

irritability,  239 

karyokinesis,  242 

membrane,  235,  238 

metabolism,  237,  239 

mitosis,  242 

motion,  240 

nucleus,  234,  237,  238 

origin,  236 

plasma,  253 

pigment,  253 

protoplasm,  234,  235,  237 

spontaneous  generation,  236 

wall,  238 

vital  manifestations,  239 
Cement  corpuscles,  354 

fibers,  356 
Cementification,  358,  433 
Cementoblasts,  368,  433 
Cementum,  75,  348 

calcification  of,  282,  358,  433 

canaliculi  of,  349 

fibers  of,  356 

histology  of,  348 

lacuna;  of,  349 

lamella?,  353 

matrix  of,  349 
Central  incisor,  lower,  165 

upper,  102 
Centrosome,  237 
Cervical  line,  80 
Cheeks,  8 

blood  supply  to,  13 

external  covering  of,  9 

glands  of,  9,  300,  307 

integument  of,  9 

internal  covering  of,  9 

mucous  membrane  of,  9,  285 

muscles  of,  9 

muscular  tissue  of,  35 

nerves  of,  1 5 

substance  of,  9 
Chromatin  substance,  238 
Chromosomes,  242 
Circulatory  organs,  274 
Circumvallate  papillae,  27 
Close  skein,  242 
Coelom,  251 

Concrescence  of  teeth,  440 
Condyle,  neck  of,  59 
Condyloid  process,  59-66 

forms  of,  66 
Connective  tissue,  cells  of,  252 

classification  of,  252 


Connective  tissue,  fibrous,  252 

intercellular  substance  of,  252 
origin,  250 

Coronoid  process,  59 

Crowns,  anomalous,  438 

Cuspid,  lower,  169 
upper,  116 

Cytoplasm,  237 


Decalcification,  441 
Deciduous  lower  central  incisor,  226 
cuspid,  227 
first  molar,  223,  228 
lateral  incisor,  220,  226 
lateral  incisor,  measurement,  219 
second  molar,  225,  230 
teeth,  decalcification  of,  219,  290 
detail  description  of,  219 
enamel  organs  for,  394 
general  description  of,  216 
occlusion  of,  218 

pulp  chambers  and  canals  of,  230 
upper  central  incisor,  219 
cuspid,  221 
Deep  facial  artery,  18,  23 

branches  of,  23 
Dendrites,  262 
Dental  arch,  80 

arrangement  of  teeth  in,  81 
curve  described  by,  85 
influence  of  temperament  on,  86 
follicle,  393,  409 
formula,  78 
Dental  furrow,  415 

Pulp,  359 

sacculus,  409 
Dentin,  75,  331 

calcification  of,  340 

cells,  408,  409 

chemical  analyses  of,  332 

exposed  by  dissection,  417 

fibers  of,  331,  335 

granular  layer,  339 

history  of,  331 

matrix  of,  332 

organ,  393,  407 

papillae,  340,  408 

tubules  of,  331,  333 
Dentinal  fibers,  335 

sheaths,  331,  334 

tubules,  331,  333 
walls  of,  334 
Dentinification,  340 
Dentition,  completion  of,  441 

lack  of,  453 
Depressor  anguli  oris,  12 

labii  inferioris,  7 

labii  superioris,  6 
Deutoplasm,  238 
Development  of  enamel,  321 

of  permanent  teeth,  427 

of  teeth,  391,  442 
Diaster  stage,  242 
Digastric  fossa,  56 


INDEX 


463 


Duct,  parotid,  303 
sublingual,  304 
submaxillar)-,  303 

Ectoderm,  247 
Elastic  cartilage,  254 
Embryology,  general,  233,  240,  244 

of  mouth  and  teeth,  374 
Enamel,  75,  313 

ameloblasts  of,  323 
brown  striae  of,  318 
calcification  of,  324 
cells,  324,  394 
chemic  composition  of,  314 
cuticle  of,  364 
development  of,  321 
histology  of,  313 
organ,  361,  393,  394 
cells  of,  394,  401 
external  epithelium  of,  322,  394 
form  of,  394.  396 
internal  epithelium  of,  322,  394 
stellate  reticulum  of,  322,  394 
stratum  intermedium  of,  322,  394 
prisms  of,  314,  325 
rods,  formation  of,  314 
Endocardium,  278 
Endomysium,  261 
Endoplasm,  238 
Endothelium,  275 
Entoderm,  247 
Epicardium,  278 
Epimysium,  261 
Epithelial  cells,  249 

tissues,  234,  249 
Eruption  of  the  teeth,  433 
Erythroblasts,  270 
Erythrocytes,  270 
number,  269 
Exoplasm,  238 
External  maxillary  artery,  13 
oblique  line,  55 
pterygoid,  73 

Facial  angle,  91 

artery,  15,  32 
Facial  artery,  branches  of,  13 

nerve,  15 

branches  of,  15 
Falciform  papillae,  27 
Fat  cells,  253 
Fauces,  anterior  pillars  of,  20 

isthmus  of,  20 

pillars  of,  20 

posterior  pillars  of,  20 
Fibers,  237 

dentinal,  335 

Sharpey's,  370 
Fibroblasts,  369 
Fibro-cartilage,  254 
Fibrous  connective  tissue,  252 
Fifth  nerve,  99 

•      division  of,  99 
Filiform  papilla',  27 


Floor  of  the  mouth,  boundaries  of,  25 

embryology  of,  377 

framework  of,  25 
Follicle,  dental,  393,  409 
Foramen,  anterior  palatal,  18 

apical,  description  of,  190 

caecum,  27 

incisive,  44 

infra-orbital,  38,  39 

mental,  56 
Foramina,  accessory  palatal,  iS 

posterior  palatal,  18 
Fossa,  glenoid,  65 
Frenae  of  the  mouth,  295 
Fungiform  papillae,  27 
Fusion  of  teeth,  456 

and  concrescence,  456 

complete,  457 

partial,  457 


Ganglion,  Gasserian,  98 

Meckel's,  19 

sphenopalatal,  19 
Geminous  teeth,  458 
Genial  tubercles,  56 
Geniohyoglossus,  29 
Germinal  cells,  244 

layers,  247 
Germs  for  permanent  molars   409 
Gills,  374 
Gingivae,  17 
Gingival  border,  17 

margins,  outlines  of,  295 
Gland  cells,  253 

follicles,  305 

parotid,  301,  310 

saccular,  305 

sublingual,  304,  311 

submaxillar}-,  303,  310 

tubular,  305 
Glands  and  ducts  of  the  mouth,  histology  of, 

3°5 
buccal,  300,  307 

excretory  ducts  of,  306 
ductless,  282 
labial,  293,  300 
lingual,  301,  305 
molar,  300,  306 
of  the  cheeks,  307 
of  the  hard  palate,  307 
of  the  mouth,  300,  305 
of  the  soft  palate,  301,  307 
palatal,  301,  307 
salivary,  301,  309 
Glenoid  fossa,  65 
Gomphosis,  76 
(Jranules,  272 

acidophilic,  272 
basophylic,  272 
neu  trophy  lie,  272 
Groove,  infra-orbital,  39 
Ground  substance,  234 
Gubernaculum,  423 
foramina  of,  429 


464 


INDEX 


Gums,  294 

epithelium  of,  288 
fibrous  tissue  of,  288 
general  des<  ription  of,  204 
mucous  membrane  of,  2S7,  295 

Hard  palate,  16 
arch  of,  17 

blood  supply  to,  18 

bones  of,  17 

covering  of,  18 

formation  of,  18 

glands  of,  17,  307 

mucous  membrane  of,  17,  289 

nerves  of,  19 
Hare-lip,  449 
Haversian  canals,  254 
Heart,  278 
Hematoblasts,  270 
Hemoglobin,  269 
Heredity,  242 
Highmore,  antrum  of,  48 
Histogenesis,  233,  244,  247 
Histology,  233 
Hooke,  Robert,  234 
Hyaline  cartilage,  313 
Hyaloplasm,  238 
Hyoglossus,  28 
Hyoid  bone,  62 

development,  63 

greater  cornua,  62 

lesser  cornua,  62 

muscles  attached  to,  63 
Hypoblast,  247 
Hypophysis,  282 

Incisive  foramen,  44 

fossa,  39,  52 
Incisor  crest,  44 

lower  central,  165 

lateral,  165 
upper  central,  102 
lateral,  no 
Inferior  coronary  artery,  8,  13 
vein,  13 
dental  artery,  96 

incisive  branch  of,  98 
mental  branch  of,  98 
canal,  59 
meatus,  43 
nerve,  101 
lingualis,  32 
maxillary  bone,  54 
body  of,  55 
development  of,  60 
facial  surface  of,  55 
internal  surface  of,  56 
muscles  attached  to,  60 
vertical  portion  of,  58 
palatal  vein,  18 
turbinated  crest,  40,  43 
Infra-orbital  canal,  39 
foramen,  39 
groove,  39 
Intercellular  substance,  251 


Interglobular  spaces,  339 

Internal  maxillary  artery,  18,  94 

infra-orbital  branch  of  the,  94 
superior  maxillary  branch  of  the,  94 
oblique  line,  56 
pterygoid,  73 

Interproximate  spaces,  86 

Intima,  276 

Involuntary  muscular  tissue,  260 

Isthmus  of  fauces,  20 

Labial  glands,  300,  306 
Lacrimal  canal,  41 

tubercle,  41 
Lacunae,  256 
Lateral  incisor,  upper,  105 

lower,  168 
Layer,  granular,  339 
Malpighi's,  443 
Leucocytes,  271 

large  mononuclear,  272 
small  mononuclear,  272 
polymorphonuclear,  272 
Dolynuclear,  272 
transitional,  272 
Levator  akeque  nasi,  4 
anguli  oris,  n 
labii  inferioris,  6 

superioris,  6 
palati,  22 
Lines  of  Schreger,  320 
Lingual  artery,  32 
glands,  300,  342 
vein,  33 
Lips,  2 

blood  supply  to,  7 
external  covering  of,  3 
frenae  of,  3,  295 
glands  of,  3,  300,  306 
integument  of,  3 
internal  covering  of,  3 
mucous  membrane  of,  3,  284 
muscles  of,  3 
muscular  tissue  of,  34 
nerves  of,  8 
substance  of,  3 
Loose  skein,  242 
Lower  bicuspid,  first,  172 

buccal  surface  of,  173 
calcification  of,  172 
cusps  of,  172 
distal  surface  of,  174 
lingual  surface  of,  174 
measurements  of,  172 
mesial  surface  of,  174 
neck  of,  174 
occlusal  surface  of,  172 
pulp  cavity  of,  213 
root  of,  175 
second,  175 

buccal  surface  of,  176 
calcification  of,  175 
distal  surface  of,  178 
lingual  surface  of,  177 
measurements  of,  175 


INDEX 


465 


Lower  bicuspid,  second,  mesial  surface  of ,  177 
neck  of,  178 

occlusal  surface  of,  175 
root  of,  178 
bicuspids,  general  description  of  the,  172 

pulp  cavities  of,  213 
cuspid,  169 

calcification  of,  169 
cusp  of,  171 
distal  surface  of,  170 
labial  surface  of,  169 
lingual  surface  of,  170 
measurements  of,  169 
mesial  surface  of,  170 
neck  of,  171 
root  of,  171 
cuspids,  pulp  cavities  of,  212 
incisor,  central,  165 

calcification  of,  165 
cervical  margin  of,  167 
cutting-edge  of,  167 
distal  surface  of,  167 
labial  surface  of,  165 
lingual  surface  of,  165 
measurements,  165 
mesial  surface  of,  166 
neck  of,  168 
root  of,  168 
incisors,  pulp  cavities  of,  211 
lateral  incisor,  general  description  of,  168 
molar,  first,  178 

buccal  surface  of,  1S1 
calcification  of,  178 
cusps  of,  180 
distal  surface  of,  183 
lingual  surface  of,  182 
measurements  of,  178 
mesial  surface  of,  182 
neck  of,  183 
occlusal  surface  of,  178 
roots  of,  183 
second,  184 

buccal  surface  of,  185 
calcification  of,  184 
distal  surface  of,  187 
lingual  surface  of,  186 
measurements  of,  184 
mesial  surface  of,  186 
occlusal  surface  of,  184 
roots  of,  187 
third,  calcification  of,  187 

general  description  of  the,  187 
roots  of,  188 
types  of  the,  189 
molars,  pulp  cavities  of,  2 14 
Lymph,  266 
cells,  267 
corpuscles,  267 
development,  267 
plasma,  267 
vessels,  267 
Lymphocytes,  271 
Lymphoid  cells,  281 
organs,  281 
tissue,  2 Si 

3° 


Malpighi's  layer,  443 
Mandible,  54 

evolution  of,  412 
Masseter,  n,  92 
Mastication,  muscles  of,  71 

active  organs  of,  1 
Matrix,  251 
Maxillary  bones,  development  of,  411 

sinus,  48 
superior,  37 
Meckel's  cartilage,  62,  384,  412 

ganglion,  19 
Media,  276 

Median  raphe,  27,  297 
Membrana  eboris,  361,  408 

praeformativa,  325 

propria,  253 
Mental  foramen,  56 

protuberance,  55 
Mesoderm,  247 

parietal,  251 

visceral,  251 
Mesothelium,  275 
Metaplasm,  238 
Microsomes,  231 
Middle  meatus,  43 

superior  dental  nerve,  100 
Migrator}'  activity,  268 

cells,  268 
Moral  glands,  307 

lower,  first,  178 
second,  184 
third,  187 

upper,  first,  138 
second,  150 
third,  157 
Monaster  stage,  242 
Morula,  245 
Motion,  240 

amoeboid,  240 

ciliary,  240 

contractile,  240 
Mouth,  1 

angles  of,  2 

bones  of,  37 

boundaries  of,  1 

contents  of,  1 

development,  411 

dissection  of,  15 

divisions  of,  15 

entrance  to,  1 

epithelium  of,  283 

floor  of,  16,  24 

frenac  of,  295 

general  description  of,  1 

glands  of,  297,  305 

inferior  portion  of,  15 

interior  of,  15 

lateral  walls  of,  8 

mucous  membrane  of,  283,  296 

muscular  tissues  of,  34 

posterior  boundary  of,  20 

primitive,  374 

roof  of,  15,  16 

situation  of,  1 


j.66 


INDEX 


Mouth,  submucosa  of,  284 
superior  portion  of,  15,  16 
tunica  propria  of,  284 
vestibule  of,  1 

walls  of,  8 
Mucous  membrane  of  the  cheeks,  histology 
of,  286 
of  gums,  histology  of,  287 
of  lips,  histology  of,  284 
of  mouth,  283,  296 
blood  supply  to,  284 
histology  of,  284 
nerve  supply  to,  284 
of  tongue,  histology  of,  290 
Muscle,  external  pterygoid,  73 
internal  pterygoid,  73 
masseter,  n,  71 
temporal,  72 
Muscles,  angular  series,  20 
of  mastication,  71 
of  soft  palate,  20 
of  tongue,  28 
Muscular  tissue,  258 
tissues  of  cheek,  35 

endomysium  of,  260 
involuntary,  260 
of  lips,  34  _ 

perimysium  of,  260 
sarcolemma  of,  260 
sarcoplasm  of,  260 
of  mouth,  histology  of,  34 

non-striated,  260 
of  soft  palate,  35 

striated,  260 
of  tongue,  36 
voluntary,  260 
Mylohyoid  ridge,  56 
Myocardium,  278 
Myrtiform  fossa,  39 

Nasal  crest,  44 

spine,  44 
Nasmyth's  membrane,  348,  364 
Nerve,  anterior  palatal,  19 

cell,  261 

corpuscles,  264 

fiber,  axis  cylinder  of,  264 
neurilemma  of,  264 

Ibers,  264 

medullated,  265 
non-medullated,  265 

inferior  dental,  101 
maxillary,  101 

middle  superior  dental,  100 

posterior  superior  dental,  100 

process,  263 

superior  dental,  100 

superior  maxillary,  99 
Nerves,  264 

endoneurium  of,  265 

epineurium  of,  265 

funiculi  of,  265 

medullar}'  sheath  of,  265 

perineurium  of,  265 

of  tongue,  33 


Nerves,  system  of,  262 

Nervous  tissue,  261 

Neumann's  sheath,  334 

Neurilemma,  264 

Neuroblasts,  262 

Neurocyte,  261 

Neuron,  261 

Neutrophils,  273 

Non-striated  muscular  tissues,  26c 

Nuclear  membrane,  238,  247,  307 

reticulum,  238 

stain,  236 
Nucleolus,  239 
Nucleoplasm,  237 
Nucleus,  234,  237,  238 

Occlusiox  of  the  teeth,  88 

of  the  deciduous  teeth,  218 
Odontoblastic  cells,  408 
Odontoblasts,  342,  361,  408 

processes  of,  374 
Oral  cavity,  374 

embryology  of,  374 

plate,  374 

sinus,  374 
Orbicularis  oris,  4 
Organogenesis,  233 
Organs  and  tissues,  243 
Osteoblasts,  317,  367 
Osteoclasts,  369 
Overbite,  91 
Ovum,  236,  244 

fertilization,  244 

maturation,  245 

segmentation,  245 

Palatal  glands,  301,  307 
raphe,  16,  297 
ruga?,  16,  297 
Palate,  bone,  articulation  of,  53 

attachment  of  muscles  to,  53 
blood  supply  to,  53 
development  of,  53 
horizontal  plate  of,  51 
vertical  plate  of,  51 
bones,  50 
hard,  15,  16 
soft,  19 
Palatoglossus,  20 
Palatomaxillary  suture,  18 
Palatopharyngeus,  22 
Papillae  of  the  tongue,  27 
Paraplasm,  238 
Parotid  duct,  303 

_  gland,  301,  309,  345 
Perichondrium,  3T3 
Perimysium,  261 
Periosteum,  258 

Permanent  incisors,  papilla-  for,  429 
molars,  germs  for,  409 
teeth,  76 

advance  of,  441 

preparation  for  development  of,  339 
Phagocytes,  240,  271 
Pigment  cells,  253 


INDEX 


467 


Pillars  of  fauces,  20 
Plasma  cells,  253 
Posterior  foramina,  18 
palatal  canal,  41 
superior  dental  nerve,  99 
Primitive  dental  furrow.  4:5 
Process,  alveolar,  44 
condyloid,  59 
coronoid,  59 
fronto-nasal,  374 
Protoplasm,  234,  237 
Protoplasmic  stains,  236 
Pulp,  blood-vessels  of,  363 
canals,  190 

cavities,  description  of,  190 
dissections  to  show,  190 
horns  of,  191 
of  deciduous  teeth,  230 
of  lower  teeth,  211 
of  teeth,  190 
of  upper  teeth.  192 
cells  of,  342 
histology  of,  342 
nerves  of,  363 
odontoblasts  of,  360 

Quadratus  menti,  7 

Ranixe  artery,  32 
Raphe,  16,  27,  297 
Retzius,  brown  striae  of,  318 
Ridge,  mylohyoid,  56 
Risorius  muscle,  12 
Roots,  anomalous,  453 
formation  of,  433 
Rouleaux,  269 
Ruga;,  16,  297 


Sacculus,  dental,  247,  409 
Salivary  glands,  301,  309 

blood-vessels  of,  311 

nerves  of,  311 
Sarcolemma,  260 
Sarcoplasm,  260 
Schleiden,  234 
Schreger,  lines  of,  358 
Schultze,  Max,  236 
Schwann,  235 

sheath  of,  325 

white  substance  of,  325 
Seventh  nerve,  15 
Sharpey's  fibers,  37 
Sheath,  medullary,  262 

of  Schwann,  263 
Simple  saccular  glands,  305 

tubular  glands,  305 
Soft  palate,  19 

blood  supply  to,  23 

glands  of,  307 

muscles  of,  20 

muscular  tissue  of,  35,  2I 

nerves  of,  23 

substance  of,  20 


Spaces,  interglobular,  339 

interproximate,  86 
Spermatozoon,  244 
Sphenomaxillary  ligament,  67 
Sphenopalatal  ganglion,  19 
Spongioplasm,  238 
Stains,  272 
acid,  272 
basic,  272 
neutral,  272 
Stellate  reticulum,  322,  394 
Stenson,  foramen  of,  44 
Stratum  intermedium,  322,  394 
Striated  muscular  tissue,  319 
Styloglossus,  31 
Stylomaxillary  ligament,  68 
Sublingual  artery,  32 
duct,  304 
fossa,  56 
gland,  303 
Submaxillar}-  duct,  303 
fossa,  56 
gland,  303 
Sub  mucosa,  284 
Superior  coronary  artery,  8,  13 
vein,  13 
lingualis,  32 

maxillary  bone,  articulation  of,  47 
alveolar  process  of,  44 
blood  supply  to,  47 
development  of,  47 
facial  surface  of,  38 
malar  process  of,  43 
muscles  attached  to,  47 
nasal  process  of,  42 
orbital  surface  of,  37 
palatal  process  of,  40,  43 
proximal  surface  of,  40 
sinus  of,  48 
tuberosity  of,  42 
zygomatic  surface  of,  42 
bones,  37 
meatus,  43 
nerve,  96 
palatal  vein,  18 
turbinated  crest,  43 
Supernumerary  teeth,  453 
Suture,  palatomaxillary7,  18 
Symphysis,  55 

Teeth,  75 

anterior,  79 

apical  extremities  of,  79 
articulation  of,  88 
attachment  of,  75 
blood  supply  to,  94-98 
classification  of,  75,  76 
complex,  75 
deciduous,  76,  216 

description  in  detail  of,  102 
development  of,  391,  442 
dissections  of,  190 
division  of,  75 
eruption  of,  289 
names  of,  78 


468 


INDEX 


Teeth,  nerve  supply  to,  98 

occlusion  of,  88 

permanent,  76 

posterior,  79 

pulp  cavities  of,  190 

roots  of,  78 

simple,  75 

surfaces  of,  78 

tissues  of,  313 

veins  from,  98 
Temporal  muscle,  72 
Temporomandibular  articulation,  64 

movements  of,  69 
Tensor  palati,  23 
Thyroid  bodies,  282 
Tissue,  adipose,  253 

areolar,  252 

connective,  234,  250 

elementary,  234,  243 

epithelial,  234,  249 

muscular,  234,  258 

nervous,  234,  261 
Tissues  of  the  body,  divisions  of,  234 

of  the  teeth,  313 
Tongue,  25 

attachment  of,  26 

base  of,  26 

blood-vessels  of,  32 

circumvallate  papillae  of,  291 

dorsum  of,  26 

filiform  papillae  of,  290 

frenum  of,  27 

function  of,  26 

fungiform  papillae  of,  291 

glands  of,  301,  308 

median  raphe  of,  27 

mucous  membrane  of,  290 

muscles  of,  28 

muscular  tissue  of,  36 

nerves  of,  33 

papillae  of,  27 

post-tip,  26 

prebase  of,  26 

shape  of,  26 

size  of,  26 

substance  of,  26 
Tonsil,  20 
Tonsillar  recess,  20 
Tooth  band,  392 

development,  cellular  stage  of,  391,  442 
saccular  stage  of,  411,  417 

follicle,  walls  of,  410 

fundamental  parts  of  a,  75 
general  description  of  a,  72,  75 

germs,  393 

roots,  preparations  for  development  of, 

433 
sac,  409 

sacs  exposed  by  dissection,  415 
sockets,  45 
tissues  of  a,  75 
Transverse  facial'artery,  13 
branches  of,  13 
vein,  13 
Tunica  propria,  328 


Upper  bicuspid,  first,  125 
angles  of,  130 
buccal  surface  of,  128 
calcification  of,  125 
crown  of,  126 
cusps  of,  127 
distal  surface  of,  130 
lingual  surface  of,  129 
measurements  of,  125 
mesial  surface  of,  129 
neck  of,  130 
occlusal  surface,  126 
pulp  cavity  of,  198 
roots  of,  131 
types  of,  132 

second,  calcification  of,  135 
general  description  of,  136 
measurements  of,  135 
pulp  cavities  of,  201 
cuspid,  116 

calcification  of,  116 

crown,  117 

cusp  of,  121 

cutting-edge  of,  121 

deciduous,  229 

distal  surface  of,  120 

labial  surface  of,  117 

lingual  surface  of,  118 

measurement  of,  116 

mesial  surface  of,  119 

neck  of,  122 

pulp  cavity  of,  196 

root  of,  122 

types  of,  122 
incisor,  central,  102 

calcification  of,  102 
cervical  margin,  107 
crown  of,  103 
cutting-edge  of,  106 
developmental  grooves,  102 
distal  surface  of,  106 
labial  surface  of,  103 
lingual  surface  of,  104 
measurements  of,  102 
mesial  surface  of,  105 
neck  of,  107 
occlusal  surface,  107 
pulp  cavity  of,  192 
root  of,  108 
types  of,  108 

lateral,  no 
angles,  114 
calcification  of,  no 
crown  of,  111 
cutting-edge  of,  1 13 
deciduous,  220 
distal  surface  of,  113 
labial  surface  of,  in 
lingual  surface  of,  112 
measurements  of,  no 
mesial  surface  of,  113 

neck  of,  114 

pulp  cavity  of,  195 
root  of,  114 
types  of,  115 


INDEX 


469 


Upper  molar,  first,  138 

buccal  surface  of,  144 
calcification  of,  138 
cusps  of,  141 
deciduous,  223 
distal  surface  of,  146 
fossae  and  grooves  of,  143 
lingual  surface  of,  143 
marginal  ridges  of,  139 
measurements  of,  138 
mesial  surface  of,  146 
neck  of,  147 
occlusal  surface  of,  139 
pulp  cavity  of,  205 
roots  of,  147 
types  of,  148 

second,  150 
angles  of,  156 
buccal  surface  of,  154 
calcification  of,  150 
cusps  of,  152 
deciduous,  225 

deciduous,  225 

distal  surface  of,  155 
fossae  and  grooves  of,  154 
lingual  surface  of,  154 
marginal  ridges  of,  151 
measurement  of,  150 
mesial  surface  of,  155 


Upper  molar,  deciduous,  neck  of,  156 
occlusal  surface  of,  151 
pulp  cavity  of,  208 
roots  of,  156 
third,  157 

buccal  surface  of,  159 
calcification  of,  157 
cusps  of,  161 
distal  surface  of,  158 
fossae  and  grooves  of,  162 
lingual  surface  of,  159 
marginal  ridges,  161 
measurements  of,  157 
mesial  surface  of,  158 
occlusal  surface  of,  160 
pulp  cavity  of,  208 
types  of,  163 

Uvula,  20 

Vein,  inferior  palatal,  18 

lingual,  S3 

superior  palatal,  18 
Visceral  arches,  374 

furrows,  374 
Voluntary  muscular  tissue,  261 

Zygomaticus  major,  12 
minor,  7 


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